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faculty, and, in preparing a book by which this work may be
commenced, she has met the profpundest need of popular edu-
cation.
D. APPLETON & CO., Publishers,
New York.
LOCKYER'S ASTRONOMY.
ELEMENTS OF ASTRONOMY:
Accompanied with numerous Illustrations, a Colored Repre- ,
sentation of the Solar, Stellar, and Nebular Spectra,
and Celestial Charts ot the Northern
and the Southern Hemisphere.
By J. Norman Lockyer.
American edition, revised and specially adapted to the Schools
of the United States.
i-zmo. '^\'2 pages. Price, '^i.$o.
The volume is as practical as possible. To aid the student
m identifying the stars and constellations, the fine Celestial
Charts of Arago, which answer all the purposes of a costly Atlas
of the Heavens, are appended to the work — this being the only
text-book, as far as the Publishers are aware, that possesses this
great advantage. Directions are given for finding the most in-
teresting objects in the heavens at certain hours on different
evenings throughout the year. Every device is used to make
the study interesting; and the Publishers feel assured that
teachers who once try this book will be unwilling to exchange
it for any other.
D. APPLETON & CO., Publishers,
New York.
Digitized by the Internet Arcinive
in 2011 witin funding from
The Library of Congress
http://www.archive.org/details/physicalgeographOOgeik
SCIENCE PRIMERS, edited hy •
Professors ■ HuxLEY, ROSCOE, and
Balfour Stewart.
IV.
PHYSICAL GEOGRAPHY,
Smna IBnmtrs.
d
PHYSICAL
GEOGRAPHY.
BY
^^ ARCHIBALD GEIKIE, LL.D., F.R.S.,
Director of the Geological Survey of Scotland, and Murchison-Pro/essor
of Geology and Mineralogy in the University of Edinburgh.
WITH ILLUSTRATIONS.
NEW YORK:
D. APPLETON AND COMPANY,
I, 3, AND 5 BOND STREET.
I 880.
6q
CONTENTS.
ART. PAGE
I
^3
Introduction i i6
The Shape of the Earth . . . . , 17 — 26
Day and Night 27—38
The Air :—
I. What the Air is made of .... 39—44 16
II, The Warming and Cooling of the
Air 45-60
III. What happens when Air is warmed
or cooled — Wind 61 69
IV. The Vapour in the Air — Evapora-
tion and Condensation . . . 70 81
V. Dew, Mist, Clouds 82—89 "i
vr. Where Rain and Snow come from . 90 — 97 35
Summary 98 38
The Circulation of Water on the
Land : —
I. What becomes of the Rain . . . 99 — 107 39
n. How Springs are formed .... 108 — 116 42
III. The Work of Water underground . 117— 125 47
19
24
27
vi CONTENTS.
ART. PAGE
IV. How the Surface of the Earth
crumbles away 126 — 142 51
V. What becomes of the crumbled parts
of Rocks. How Soil is made , 143 — 153 cS
VI. Brooks and Rivers. Their Origin . 154 — 168 62
Summary 169 67
VII. Brooks and Rivers. Their Work . 170—182 68
VIII. Snow-fields and Glaciers .... 183 — 203 75
The Sea : —
I. Grouping of Sea and Land . . . 204 — 211 86
II. Why the Sea is Salt ..... 212—216 88
III. The Motions of the Sea .... 217 — 232 90
IV. The Bottom of the Sea .... 233 — 251 95
The Inside of THii Earth 252 — 265 102
Conclusion 266- -268 109
Questions iii
SCIENCE PRIMERS.
PHYSICAL GEOGRAPHY.
INTRODUCTION.
I. Let us suppose that it is summer-time, that you
are in the country, and that you have fixed upon a
certain day for a hoUday ramble. Some of you are
going to gather wildflowers, some to collect pebbles,
and some without any very definite aim beyond the
love of the hoHday and of any sport or adventure
which it may bring with it. Soon alter sunrise on the
eventful day you are awake, and great is your delight
to find the sky clear and the sun shining warmly. It
is arranged, however, that you do not start until after
breakfast-time, and meanwhile you busy yourselves in
getting ready all the baskets and sticks and other gear
of which you are to make use during the day. But the
brightness of the morning begins to get dimmed. The
few clouds which were to be seen at first have grown
large, and seem evidently gathering together for a storm.
And sure enough, ere breakfast is well over, the first
ominous big drops are seen falling. Vou cling to the
hope that it is only a shower which will soon be over,
2 SCIENCE PRIMERS, [introduction.
and you go on with the preparations for the journey
notwithstanding. But the rain shows no symptom of
soon ceasing. The big drops come down thicker and
faster; httle pools of water begin to form in the
hollows of the road, and the window-panes are now
streaming with rain. With sad hearts you have to
give up all hope of holding. your excursion to-day.
2. It is no doubt very tantalizing to be disappointed
in this way when the promised pleasure was on the
very point of becoming yours. But let us see if we
cannot derive some compensation even from the bad
weather. Late in the afternoon the sky clears a little,
and the rain ceases. You are glad to get outside
again, and so we all sally forth for a walk. Streams of
muddy water are still coursing along the sloping road-
way. If you will let me be your guide, I would advise
that we should take our walk by the neighbouring
river. We wend our way by wet paths and green
lanes, where every hedgerow is still dripping with
moisture, until we gain the bridge, and see the river
right beneath us. What a change this one day's heavy
rain has made ! Yesterday you could almost count
the stones in the channel, so small and clear was the
current. But look at it now ! The water fills the
channel from bank to bank, and rolls along swiftly.
We can watch it for a little from the bridge. As it
rushes past, innumerable leaves and twigs are seen
floating on its surface. Now and then a larger branch,
or even a whole tree-trunk, comes down, tossing and
rolling about on the flood. Sheaves of straw or hay,
planks of wood, pieces of wooden fence, sometimes
a poor duck, unable to struggle against the current,
roll past us and show how the river has risen above
INTRODUCTION.] PHYSICAL GEOGRAPHY.
its banks and done damage to the farms higher up its
course.
3. We linger for a while on the bridge, watching this
unceasing tumultuous rush of water and the constant
variety of objects which it carries down the channel.
You think it was perhaps almost worth while to lose
your holiday for the sake of seeing so grand a sight as
this angry and swollen river, roaring and rushing with
its full burden of dark water. Now, while the scene
is still fresh before you, ask yourselves a few simple
questions about it, and you will find perhaps addi-
tional reasons for not regretting the failure of the
promised excursion.
4. In the first place, where does all this added mass
of water in the river come from ? You say it was the
rain that brought it. Well, but how should it find its
way into this broad channel ? Why does not the rain
run off the ground without making any river at all ?
5. But, in the second place, where does the rain
come from ? In the early morning the sky was bright,
then clouds appeared, and then came the rain, and you
answer that it was the clouds which supplied the rain.
But the clouds must have derived the water from some
source. How is it that clouds gather rain, and let it
descend upon the earth ?
6. In the third place, what is it which causes the rive*-
to rush on in one direction more than another ? When
the water was low, and you could, perhaps, almost
step across the channel on the stones and gravel, the
current, small though it might be, was still quite per-
ceptible. You saw that the water was moving along
the channel always from the same quarter. And now
when the channel is filled with this rolling torrent of
4 SCIENCE PRIMERS. [introduction.
dark water, you see that the direction of the current is
still the same. Can yoji tell why this should be ?
7. Again, yesterday the water was clear, to-day it
is dark and discoloured. Take a little of this dirty-
looking water home with you, and let it stand all night
in a glass. To-morrow morning you will find that it
is clear, and that a fine layer of mud has sunk to the
bottom. It is mud, therefore, which discolours the
swollen river. But where did this mud come from?
Plainly, it must have something to do with the heavy
rain and the flooded state of the stream.
8. Well, this river, whether in shallow or in flood, is
always moving onward in one direction, and the mud
which it bears along is carried towards the same point
to which the river itself is hastening. While we sit on
the bridge watching the foaming water as it eddies
and whirls past us, the question comes home to us
— what becomes of all this vast quantity of water and
mud?
9. Remember, now, that our river is only one of
many hundreds which flow across this country, and
that there are thousands more in other countries
where the same thing may be seen which we have
been watching to-day. They are all flooded when
heavy rains come ; they all flow downwards ; and all
of them carry more or less mud along with them.
10. As we walk homewards again, it will be well to
put together some of the chief features of this day's
experience. We have seen that sometimes the sky
is clear and blue, with the sun shining brightly and
warmly in it ; that sometimes clouds come across
the sky, and that when they gather thickly rain is apt
to fall. We have seen that a river flows ; that it is
INTRODUCTION.] PHYSICAL GEOGRAPHY. 5
swollen by heavy rain, and that when swollen it is apt
to be muddy. In this way we have learnt that there
is a close connection between the sky above us and
the earth under our feet. In the morning, it seemed
but a little thing that clouds should be seen gathering
overhead ; and yet, ere evening fell, these clouds led
by degrees to the flooding of the river, the sweeping
down of trees, and fences, and farm produce ; and it
might even be to the destruction of bridges, the inun-
dation of fields and villages and towns, and a large
destruction of human life and property.
11. But perhaps you live in a large town and have
no opportunity of seeing such country sights as I have
been describing, and in that case you may naturally
enough imagine that these things cannot have much
interest for you. You may learn a great deal, how-
ever, about rain and streams ^ even in the streets of a
town. Catch a little of the rain in a plate, and you
will find it to be so much clear water. But look at it
as it courses along the gutters. You see how muddy
it is. It has swept away the loose dust worn by wheels
and feet from the stones of the street, and carried it
into the gutters. Each gutter thus becomes like the
flooded river. You can watch, too, how chips of
straw, corks, bits of wood, and other loose objects
lying in the street are borne away, very much as the
trunks of trees are carried by the river. Even in a
town, therefore, you can follow how changes in the sky
lead to changes on the earth.
12. If you think for a little, you will recall m.any
other illustrations of the way in which the common
things of everyday life are connected together. As far
back as you can remember, you have been familiar with
6 SCIENCE PRIMERS, [introduction.
such things as sunshine, clouds, wind, rain, rivers, frost,
and snow, and they have grown so commonplace that
you never think of considering about them. You cannot
imagine them, perhaps, as in any way different from
what they are ; they seem, indeed, so natural and so
necessary that you may even be surprised when anyone
asks you to give a reason for them. But if you had
lived all your lives in a country where no rain ever
fell, and if you were to be brought to such a
country as this, and were to see such a storm of rain
as you have been watching to-day, would it not be
very strange to you, and would you not naturally
enough begin to ask the' meaning of it ? Or suppose
that a boy from some very warm part of the world
were to visit this country in winter, and to see for the
lirst time snow fall, and the rivers solidly frozen-over,
would you be surprised if he showed great astonish-
ment ? If he asked you to tell him what snow is, and
why the ground is so hard, and the air so cold, why
the streams no longer flow, but have become crusted
with ice — could you answer his questions ?
13. And yet these questions relate to ver}' common,
everyday things. If you think about them, you will
learn, perhaps, that the answers are not quite so easily
found as you had imagined. Do not suppose that
because a thing is common, it can have no interest for
you. There is really nothing so common as not to
deserve your attention, and which will not reward you
for your pains.
14. In the following pages I propose to ask you to
look with me at some of these common things. You
must not think, however, that it is my wish merely to
set you certain lessons which you have to learn, and
INTRODUCTION.] PHYSICAL GEOGRAPHY. 7
to give you some rudiments of knowledge which you
must commit to memory. I would fain have you not
to be content with what is said in this little book, or
in other books, whether small or great, but rather to
get into the habit of using your own eyes and seeing
for yourselves what takes place in this wonderful
world of ours. All round you there is abundant
material for this most delightful inquiry. No excur-
sion you ever made in pursuit of mere enjoyment and
adventure by river, heath, or hill, could give you more
hearty pleasure than a ramble with eyes and ears alike
open to note the lessons to be learnt from every day
and from every landscape. Remember that besides
the printed books which you use at home, or at school,
there is the great book of Nature, wherein each of us,
young and old, may read, and go on reading all tlirough
life without exhausting even a small part of what it
has to teach us.
15. It is this great book — Air, Earth, and Sea —
which I would have you look into. Do not be content
with merely noticing that such and such events take
place. For instance, to return to our walk to the
flooded river; do not let a fact such as a storm or
a flood pass without trying to find out something
about it. Get into the habit of asking Nature ques-
tions, as we did in the course of our homeward walk.
Never rest until you get at the reasons for what you
notice going on around you. In this way even the
commonest things will come to wear a new interest for
you. AVherever you go there will be something for
you to notice ; something that will 'serve to increase
the pleasure which the landscape would otherwise
afford. You will thus learn to use your eyes quickly
9
8 SCIENCE PRIMERS. [shape of
and correctly ; and this habit of observation will be of
the utmost value to you, no matter what may be the
path of life which lies before you.
1 6. In the following Lessons I wish to show you what
sort of questions you may put about some of the chief
parts of the book of Nature, and especially about two
of these — the Air and the Earth. Each of us should
know something about the air we breathe and the earth
we live upon, and about the relations between them.
Our walk showed us a little regarding these relations
when it enabled us to connect the destruction of fences
and farms with the formation of clouds in the sky.
Many other relations remain for you to find out. In
tracing these you are really busy with science, with
that branch of science called Physical Geography,
which seeks to describe this earth with all the move-
ments which are going on upon its surface. And yet
you are not engaged in anything very difficult or un-
interesting. You are simply watching with attentive
eyes the changes which are continually taking place
around you, and seeking to find out the meaning of
these changes, and how they stand related to each
other.
THE SHAPE OF THE EARTH.
17. Before observing what takes place on the surface
of the earth, it may be well if you form a clear notion
about the shape of the whole earth as a mass, and if
you fix in your minds some of the great leading
features of the connection between the earth and
the sun.
18. When you stand in the middle of a broad flat
country, or look out upon the wide sea, it seems to
THE EARTH.] PHYSICAL GEOGRAPHY. g
you as if this world on which we live and move were
a great plain, to the edge of which you would come if
you went far enough onward. This is the first notion
we all have as children. It was also the firm belief
of mankind in early times. The sun and moon were
then thought to rise and set only for the use of people
here ; and the sky, with all its stars, was looked upon
as a great crystal dome covering and resting upon
the earth.
19. But you can easily prove to yourselves that the
eye is deceived about the flatness of the earth, and
that what seems quite level is in reality curved. In a
wide level country, such as many parts of the midland
and eastern counties of England, you cannot see trees
and houses farther away than some four or five miles. If
you climb to the top of a church tower, you find many
objects come into sight which you could not have
seen from the ground. And if there should happen
to be a range of hills in the neighbourhood, you would
note from their tops a still larger number of points
which before were hidden. The higher you climb
above the ground, therefore, the further you can see.
20. Again : suppose you were at the bottom of a tall
sea-cliff, and on looking out to sea were to note the
sails of a distant ship. If you mounted to the top of
the cliff, you might see not only the sails, but the
whole vessel, and your eye would probably pick out
ships still further away, appearing as mere specks
along the line of meeting between sea and sky, and
which you could not see at all from the beach.
21. Suppose further, that you were to sit down on the
top of that cliff, and watch these vessels for a time.
Some of them, which at first were so far away that
lO
SCIENCE PRIMERS.
[shape of
they could hardly be seen, would probably seem to
grow bigger and clearer. You would begin to make
out the tops of the masts and sails ; by and by the
rest of the sails would appear, until at last the hulls
too came into sight. These vessels would seem' to
you to have sailed up over what used to be thought
the edge of the world.
Fig. I. — Disappearance of a Ship at Sea owing to the curved surface of
the Earth.
22. On the other hand, some of the ships which were
near you at first will gradually sail away towards the
same distant parts. Their hulls will dip .down into
the sea, as it were ; then the sails will slowly sink, and
in the end all trace of the vessels will have vanished.
23. Now, in making these observations, you will have
gathered facts which prove that the world we live in
is not a flat plain, but has a curved surface, or in other
words is a globe. To use your eyes in this way, and
seek out the meaning of that which you see, would
neither be a hard nor a dull task ; and yet you would
really be engaged in what is called observational
science. When you watch how the ships at sea appear
THE EARTH.] PHYSICAL GEOGRAPHY. ii
to you as they come and go, you observe facts. When
you put the facts together, and reason out their con-
nection and meaning, and find that they prove the
roundness of the earth, you make an induction or
inference from them. Now it is this union of obser-
vation and induction which makes science.
24. You may observe, then, and prove that the old
and natural-enouerh notion about the flatness of the
earih is quite untrue ; and that, flat as the sea and
land may appear, they are only parts of a great curve.
If youwere to set sail from England, and keep sailing
on in the same general direction without turning back,
you would in the end come to England again. You
would sail round the world, and prove it to be actually
a globe. Now, this has often been done. Many voy-
ages have been made round the world, and, instead
of coming to its edge, the voyagers, or " circumnavi-
gators," as they are called, have always found the
land and sea to wear the same curved surface which
we can see for ourselves at home.
25. Though you may find it easy enough to believe
that the surface of the earth is part of a curve when
you look out upon the broad sea, yet when you see
a landscape where the ground is very uneven, such,
for example, as a region of high mountains and deep
valleys, you may find perhaps some difficulty in under-
standing how it can possibly be that such an irregular
surface can be spoken of as part of a curve. In
reality, however, the earth is so big, that even the
highest mountains are in comparison merely like little
grains on the surface. It is only when the surface is
level, as on a great plain or on the sea, that we can
usually judge by the eye as to the real form of the
12
SCIENCE PRIMERS.
[the earth.
earth. But even in the most rugged ground the curve
is there, though we may fail to notice it.
26. But the curve, after all, is a very gentle one.
You can see the vessels at sea for many miles before
they sink down out of sight. The fact that the curve
is so gentle shows that the circle of which it forms
Fig. 2.— The Earth and Moon as they would appear seen from the Sun.
part must be of great size. Now, it has been nieasured
by astronomers, and found to be so big that if a rail-
way train could go completely round the earth at a
rate of thirty miles an hour without stoppage, it would
take more than a month to complete the circuit.
DAY, ETC.] PHYSICAL GEOGRAPHY. 13
DAY AND NIGHT.
27. Day by day, as far back as you can remember,
you have been accustomed to see the sun travel across
the sky. Night after njght, when the air has been free
from cloud, you have seen the moon and stars sailing
slowly overhead. You cannot be more confident of
anything than you are that the sun will appear again
to-morrow, and move on from year to year as it has
done in the past. You have seen that a slow, regular,
and unceasing motion seems to be going on all round
the earth. Have you ever wondered what can be the
cause of this motion ?
28. When the sun shines it is warm, when clouds
obscure the sky the air is more chilly, and at night,
when the sun does not shine at all, we feel a sensation
of cold. Again : by day the sky is filled with light,
but when the sun sinks in the west darkness begins.
You see from this that we depend upon the sun for
light and heat. It is evident that we cannot properly
understand what takes place upon the earth until we
learn something about the relations of the earth to
the sun.
29. Perhaps your first impression has been like that of
mankind in general long ago. They believed the earth
to remain as the fixed central point of the universe,
round which sun, moon, and stars were ceaselessly
revolving. To this day we speak of these heavenly
bodies as rising and setting, as if we still regarded
them as performing a journey round the earth.
30. But instead of being the centre of the universe
our earth is in reality only one of a number of heavenly
14 SCIENCE PRIMERS. [day and
bodies which travel unceasingly round the sun. The
sun is the great central hot mass which warms and
lights the earth, and round which the earth is con-
tinually circling.
31. The succession of day and night seems to be
owing to the movements of the sun, but in reality it is
caused by the turning or rotation of the earth itself.
You can readily illustrate this. Set a humming-top
spinning as rapidly as you can. It seems to stand
for a while motionless upon its point, but actually it
is rotating with great rapidity. Imagine a line passing
straight up from the point below, to the top of the
stalk above. Every part of the top is spinning round
this central line, which is called the axis of rota-
tion. In the same kind of way the earth is spinning
rapidly on its axis.
32. Again : take an ordinary school-globe, and place
a lighted candle a few feet from it, in a line with the
brass circle. You can make the globe turn round on
its axis. Whether it is allowed to remain at rest or
is sent spinning round rapidly, the half of it next the
candle is lighted, and the other half away from the
candle is in shade. When it is at rest, the places
marked on one side remain in the light, while those
on the opposite side remain in the dark. As you turn
it round, each place in succession is brought round to
the light, and carried on into the shade again. And
while the candle remains unmoved, the rotation of
the globe brings alternate light and darkness to each
part of its surface.
33. Instead of the little school-globe in this illustra-
tion, imagine our earth, and in place of the feeble
candle, the great sun, and you will see how the rotation
NIGHT.] PHYSICAL GEOGRAPHY. 15
of the earth on its axis must bring alternate light and
darkness to every country.
34. You must not suppose that there is any actual
rod passing through the earth to form the axis round
which it turns. The axis is only an imaginary line,
and the two opposite points where it reaches the sur-
face, and where the ends of the rod would come out
were the axis an actual visible thing, are called the
North Pole and the South Pole. They are
represented by the two little points by which the
school-globe is fixed in its place.
35. Round this axis the earth spins once in every
twenty-four hours. All this time the sun is shining
steadily and fixedly in the sky. But only those parts
of the earth can catch his light which happen at any
moment to be looking towards him. There must
always be a bright side and a dark side, just as there
was a bright side and a dark side when you placed
the globe opposite to the candle. Now you can
easily see that if there were no motion in the earth,
half of its surface would never see the light at all,
while the other half would never be in darkness. But
since it rotates, every part is alternately illuminated
and shaded. When we are catching the sun's light,
we have Day ; when we are on the dark side, we
have Night.
2i(). The sun seems to move from east to west. The
real movement of the earth is necessarily just the
reverse of this, viz. from west to east In the morn-
ing we are carried round into the sunlight, which
appears in the east. Gradually the sun seems to
climb the sky until we are brought directly opposite
to him at noon, and gradually he sinks again to set in
1 6 SCIENCE PRIMERS. [the
the west, as the earth in its constant rotation bears us
round once more into the dark. Even at night, how-
ever, we can still trace the movement of the earth by
the way in which the stars one by one rise and set, until
their lesser lights are quenched in the returning light
of another day.
37 . All the time that the earth is rotating on its axis it
is circling or revolving round the sun. This motion
is called the revolution of the earth in its orbit.
To go completely round the sun, the earth has to travel
over so wide a circle or orbit, that it takes rather
more than three hundred and sixty-five days to per-
form the journey, even though it is rushing along at
an average speed of about nineteen miles in a second.
2i^. By the motion of rotation, time is divided into
days and nights, by that of revolution it is marked off
into years. So that in this way the earth is our great
time-keeper.
THE AIR.
I. What the Air is made of.
39. When we begin to look attentively at the world
around us, one of the first things to set us thinking
is the air. We do not see it, and yet it is present
wherever we may go. At one time it blows upon us
in a gentle breeze, at another it sweeps along in a
fierce storm. What is this air?
40. Although invisible, it is yet a real, material sub-
stance. When you swing your arm rapidly up and down
you feel the air offering a resistance to the hand. The
air is something which you can feel, though you cannot
see it. You breathe it every moment. You cannot get
AIR.] PHYSICAL GEOGRAPHY. 17
away from it, for it completely surrounds the earth.
To this outer envelope of air, the name of Atmo-
sphere is given.
41. From the experiments explained in the Chemis-
try Primer (Art. 9) you learn that the air is not a simple
substance, but a mixture of two invisible gases, calle^i
nitrogen and oxygen. But besides these chief ingre-
dients, it contains also small quantities of other sub-
stances ; some of which are visible, others invisible. If
you close the shutters of a room, and let the sunlight
stream through only one chink or hole into the room,
you see some of the visible particles of the air. Hun-
dreds of little motes or specks of dust cross the beam
of light which makes them visible against the sur-
rounding darkness, though they disappear in full day-
light. But it is the invisible parts of the air which
are of chief importance j and among them there are
two which you must especially remember — the vapour
of water and carbonic acid gas. You will soon
come to see why it is needful for you to distinguish
these.
42. Now what is this vapour of water? You will
understand its nature if you watch what takes place
when a kettle boils. From the mouth of the spout a
stream of white cloud comes out into the air. It is in
continual motion; its outer parts somehow or other
disappear, but as fast as they do so they are suppHed
by fresh materials from the kettle. The water in the
kettle is all the while growing less, until at last, if you
do not replenish it, the whole will be boiled away, and
the kettle left quite dry. What has become of all
the water? You have changed it into vapour. It is
not destroyed or lost in any way, it has only passed
1 8 SCIENCE PRIMERS. [the
from one state into another, from the liquid into the
gaseous form, and is now dissolved in the air.
43. Now the air always contains more or less vapour
of water, though you do not see it, so long as it remains
in the state of vapour. It gives rise to clouds, mist,
rain, and snow. If it were taken out of the air, every-
thing would be dried up on the land, and life would
be impossible. As you learn more and more of the
changes which take place from day to day around you,
you will come to see that this vapour of water plays
a main part in them all.
44. Carbonic acid gas is also one of the invisible
substances of the atmosphere, of which, though it
forms no more than four parts in every ten thousand,
yet it constitutes an important ingredient. You will
understand how important it is when you are told that,
from this carbonic acid in the air, all the plants which
you see growing upon the land extract nearly the whole
of their solid substance (see Chemistry Primer, Art.
11). When a plant dies and decays, the carbonic acid
is restored to the air again. On the other hand, plants
are largely eaten by animals, and help to form the
framework of their bodies. Animals in breathing
give out carbonic acid gas ; and when they die, and
their bodies decay, the same substance is again re-
stored to the atmosphere. Hence the carbonic acid
of the air is used to build up the structure both of
plants and animals, and is given back again when
these living things cease to live. There is a continual
coming and going of this material between the air
and the animal and vegetable kingdoms (see Che-
mistry Primer, Art. 13).
AIR.] PHYSICAL GEOGRAPHY. 19
II. The Warming and Cooling of the Air.
45. You know that though you cannot see the air you
jcan feel it when it moves. A light breeze, or a strong
gale, can be just as little seen by the eye as still air;
and yet we readily feel their motion. But even when
the air is still it can make itself sensible in another
way, viz. by its temperature (see Physics Primer^
Art. 51). For air, like common visible things, can be
warmed and cooled.
46. This warming ani5 cooling of the air is well illus-
trated by what takes place in a dwelling-house. If you
pass out of a warm room, on a winter's day, into ^^he
open air when there is no wind, you feel a sensation
of cold. Whence does this sensation come? Not
from anything you can see, for your feet, though resting
on the frozen ground, are protected by leather, and
do not yet feel the cold. It is the air which is cold,
and which encircles you on all sides, and robs you of
your heat ; while at the same time you are giving off
or radiating heat from your skin into the air (see
Physics Primer, Art. 67). On the other hand, if, after
standing a while in the chilly winter air you return
into the room again, you feel a sensation of pleasant
warmth. Here, again, the feeling does not come from
any visible object, but from the invisible air which
touches every part of your skin, and is thus robbed
of its heat by you.
47. Air, then, may sometimes be warm and some-
times cold, and yet still remain quite invisible. By
means of the thermometer (which is explained in the
Physics Primer, Art. 51), we can measure slight
changes of heat and cold, which even the most
sensitive skin would fail to detect.
3
20 SCIENCE PRIMERS. ^ ■ [the
48. Now, how is it that the atmosphere should
sometimes be warm and sometimes cold ? Where
does the lieat come from ? and how does the air take
it up ?
49. Let us return again to the illustration of the
house. In winter, when the air is keen and .frosty
outside, it is warm and pleasant indoors, because fires
are there kept burning. The burning of coal and
wood produces heat, and the heat thus given out
warms the air. Hence it is by the giving off or
radiation of the heat from some burning substance
that the air of our houses is made warmer than the air
outside.
50. Now, it is really by radiation from a heated
body that the air outside gets its heat. In sum-
mer, this air is sometimes far hotter than is usual in
dwelling-houses in winter. All this heat comes from
the sun, which is an enormous hot mass, continually
sending out heat in all directions.
51. But, if the sun is always pouring down heat upon
the earth, why is the air ever cold ? Place a screen
between you and a bright fire, and you will imme-
diately feel that some of the heat from the fireplace
has been cut off. When the sun is shining, expose
your hand to its beams for a time, and then hold a
book between the hand and the sun. At first, your
skin was warmed ; but the moment you put it in the
shade, it is cooled again. The book has cut off the
heat which was passing directly from the sun to your
hand. When the atmosphere is felt to be cold, some-
thing has come in the way to keep the sun's heat from
directly reaching us.
52. Clouds cut off the direct heat of the sun.
AIR.]
PHYSICAL GEOGRAPHY.
21
You must often have noticed the change of tem-
perature, when, after the sun has been shining
for a time, a cloud comes between it and the
earth. Immediately a feeling of chilliness is ex-
perienced, which passes off as soon as the cloud has
sailed on, and allowed the sun once more to come
out.
53. The air itself absorbs some of the sun's heat,
and the greater the thickness of air through which
that heat has to make its way, the more heat will be
Fig. 3.— Diagram showing the influence of the varying thickness of the
atmosphere in retarding the Sun's heat. a. Line of Sun's rays in the
morning, b. Line of the rays at noon. c. Line of the rays at sunset.
absorbed. Besides this, the more the rays of heat are
slanted the weaker do they become. At noon, for
example, the sun stands high in the sky. Its rays (as
at B in Fig. 3) are then nearest to the vertical, and
have also the least thickness of air to pass through
before they reach us. As it descends in the after-
noon, its rays get more and more slanted, and must
also make their way through a constantly increasing
thickness of air (as at c in the diagram). Hence the
middle of the day is much warmer than morning or
evening.
22 SCIENCE PRIMERS. [the
1
54. At night, when the sun no longer shines, its
heat does not directly wa.rm the part of the earth in
shadow. That part not only receives no heat from it,
but even radiates its heat out into the cold sky (see
Art. 59). Hence night is much colder than day.
55. Then, again, in summer the sun at noon shines
m.Qch higher in the sky with us, or more directly over-
head, than in winter. Its heat comes down less
obliquely and has less depth of air to pass through,
and hence is much more felt than in winter, when,
as you know, the sun in our part of the world never
rises high even at midday.
56. From all this it is evident that we get our sup-
plies of heat from the sun, and that anything coming
between us and the sun serves to interrupt this heat
and give us the? sensation of cold.
57. Still, if we were dependent for our warmth upon
the direct heat of the sun alone, we should be warm
only when the sun shines. A cloudy day would be
an extremely cold one, and every night as intensely
frosty as it ever is in winter. Yet such is not the
case. Cloudy days are often quite warm ; while we are
all aware that the nights are by no means always very
cold. There must be some way in which the sun's
heat is stored up, so that it can be felt even when he
is not shining.
58. Let us again have recourse to our first illus-
tration. If you place the back of a chair opposite to
the fire, you will find that it gets so hot that you can
hardly touch it. Remove the chair to a distant part
of the room, and it quickly cools. Hence a part of
the heat from the fire has been absorbed by the
wood, and again given out.
AIR.] PHYSICAL GEOGRAPHY. 23
59. In like manner in summer the ground gets
warmed \ in some parts, indeed, becoming even so hot
at times that we can hardly keep the hand upon it. In
hot countries this is felt much more than in Britain.
Soil and stones absorb heat readily, that is to say, soon
get heated, and they soon cool again. When they have
been warmed by the sun, the air gets warmed by con-
tact with them, and keeps its heat longer than they
do; so that even when at night the soil and stones
have become ice-cold, the air a little above is njbt so
chilly. On the other hand, when the surface of the
ground is cold, it cools the air next it. The ground
parts easily with its heat, and a vast amount of heat is
in this way radiated at night from the earth outward
into the cold starry space. Much more heat, however,
would be lost from this cause did not the abundant
aqueous vapour of the atmosphere (Art. 43) absorb
part of it, and act as a kind of screen to retard the
radiation. This is the reason why in hot climates,
where the air is very dry — that is, contains a small
proportion of the vapour of water — the nights are re-
latively colder than they are in other countries where
the air is moister. In like manner, clouds serve to
keep heat from escaping ; and hence it is that cloudy
nights are not so cold as those which are clear and
starry.
60. The atmosphere, then, is heated or cooled ac-
cording as it lies upon a warm or cold part of the
earth's surface ; and, by means of its aqueous vapour,
it serves to store up and distribute this heat, keeping
the earth from such extremes of climate as would
otherwise prevail.
24 SCIENCE PRLMERS. [the
III. What happens when Air is warmed
or cooled — Wind.
6 1. The air lying next to a hot surface is heated;
the air touching a .cold surface is cooled. And upon
such differences of temperature in the air the formation
of winds depends.
62. Hot or warm air is lighter than cold air. You
have learnt how heat expands bodies (Physics Primer,
Art. 49). It is this expansion of air, or the separation
of its particles farther from each other, which makes
it less- dense or heavy than cold air, where the particles
lie more closely together. As a consequence of this
difference of density, the light warm air rises, and the
heavy cold air sinks. You can easily satisfy yourselves
of this by experiment. Take a' poker, and heat the
end of it in the fire until it is red-hot. Withdraw it,
and gently bring some small bits of very light paper
or some other light substance a few inches above the
heated surface. The bits of paper will be at once
carried up into the air. This happens because the air
heated by the poker immediately rises, and its place
is taken by colder air, which, on getting warmed, like-
wise ascends. The upward currents of air grow feebler
as the iron cools, until, when it is of the same tem-
perature as the air around, they cease.
d'l. This is the principle on which our fireplaces are
constructed. The fire is not kindled on the hearth,
for, in that case, it would not get a large enough
draught of air underneath, and would be apt to go
out. It is placed some way above the floor, and a
chimney is put over it. As soon as the fire is lighted,
the air next it gets warmed, and begins, to mount, and
the air in the room is drawn in from below to take the
AIR.] PHYSICAL GEOGRAPHY. 25
place of that which rises. All the air which lies above
the burning coal gets warmer and lighter ; it therefore
flows up the chimney, carr}dng with it the smoke and
gases. You will understand that though a bright
blazing fire is a pleasant sight in winter, we do not
get all the heat which it gives out. On the contrary,
a great deal of the heat goes up the chimney ; and,
except in so far as it warms the walls, passes away and
warms the outer air.
64. What happens in a small way m our houses takes
place on a far grander scale in nature. As already
pointed out (Art. 50), the sun is the great source of
heat which warms and lightens our globe. While the
heat of the sun is passing through the air, it does very
little in the way of warming it. The heat goes through
the air, and warms the surface of the earth. You knov/
that in summer the direct rays of the sun are hot
enough to bum your face, and yet, if you put even a
thin sheet of paper over your head, enough to cut off
these rays, the sensation of burning heat at once goes
off, although the same air is playing about you all the
time.
65. Both land and water are heated by the sun's
rays, and the same change in the air then takes place
which we find also at our firesides. The layer of air
next the warmed earth becomes itself warmed. As it
thereby grows lighter it ascends, and its place is taken
by colder air, which flows in from the neighbourhood
to take its place. This flowing in of air is Wind.
66. It is easy for you now and then to watch how
wind arises. Suppose, for instance, that during the
summer you spend some time at the sea-coast. In the
morning and early part of the day a gentle wind will
26 SCIENCE PRIMERS. [the
often be noticed, blowing from the land out to sea.
As the day advances, and the heat increases, this wind
dies away. But after a while, when the day is beginning
to sink towards evening, another breeze may be noticed
springing up from the opposite quarter, and blowing
with a delicious coolness from the sea to the land.
These breezes are the result of the unequal heating
and cooling of the sea and land.
67. Let us understand how this takes place. On a
hot day you find that stones, soil, or other parts of the
land get very warm under the sun's rays ; yet if you
bathe in the sea at that time you feel its waters to be
pleasantly cool. This shows that the land becomes
more quickly hot than the sea. After such a hot day
you will find that at night the surface of the land
becomes much colder than the sea, because it parts
with its heat sooner than the sea does. By day the
hot land heats the air above it, and makes it lighter,
so that it ascends ; while the cooler and heavier air
lying on the sea flows landward as a cool and re-
freshing sea-breeze. By night this state of things
is just reversed ; for then the air which lies on the
chilled land being colder and heavier than that which
covers the warmer sea, flows seaward as a cool land-
breeze.
6%. Take a school-globe, and notice some of the
lines which are drawn round it. Midway between the
two poles you will notice a line running round the most
projecting part of the globe. This line is called the
equator. It divides the globe, as you see, into two
halves or hemispheres. Now, over the parts of the
earth which this line traverses, and for some way on
either side, the sun shines with intense heat all the
AIR.] PHYSICAL GEOGRAPHY. 27
year round. The air is constantly heated to a high
degree, and streams upwards in ascending currents.
But just as the hot air along this central belt mounts
up into the higher regions of the atmosphere, the
cooler air from north and south flows in along the
surface to supply its place. This constant streaming
of air into the equatorial regions forms what are known
as the Trade Winds. The steadiness of these winds, |
and the w^ay in which they may be counted upon in
navigation, led long ago to their being called by their
present name.
69. In our country the winds are by no means so
regular and constant. If you look at the map, and
mark the position of Britain upon the surface of the
earth, you will readily notice some obvious reasons
why our winds should be variable. To the west lies
the wide Atlantic Ocean ; to the east, beyond the
narrow and shallow North Sea, stretches the vast con-
tinental mass of Europe and Asia. Seas and lands
much colder than ours lie to the north ; others much
warmer than ours spread to the south. So that, with
so variable a surface receiving the sun's heat, we may
be quite prepared to find that sometimes a warm wind
blows from one quarter, and sometimes a cold wind
from another.
IV. The Vapour in the Air. Evaporation
and Condensation.
70. One of the most important ingredients in the
air was stated in Art. 41, to be the vapour of water.
Let us try to see, first of all, how it gets into and out
of the air. And in this case, as before, you will find
that great questions in science often admit of being
28 SCIENCE PRIMERS. [the
simply and readily illustrated by the most familiar
things.
71. In a warm room, where a good fire has been
burning all day, and a number of people have been
gathered together, you might suppose that the air must
be tolerably dry. But bring a tumbler of ice-cold
water into the room, and mark what happens to it.
You will see the outside of the glass immediately
covered with a fine film of mist. In a little while
minute drops of water will form out of this film, and
will go on growing, until, p( rhaps, some of them unite
and trickle down the side of the tumbler.
72. You may have noticed, too, that on very cold
nights the windows of sitting-rooms or crowded public
halls are apt to be found streaming with water on the
inside.
73. Now, in such cases, where does the moisture
come from ? Certainly not out of the glass. It is
derived from the vapour of water present in the air.
This word vapour is often used to describe some kind
of visible mist or fog. But these visible forms of mois-
ture are not properly vapour in the sense in which the
term is used in science. The aqueous vapour of the
air is always invisible, even when the air is saturated
with it, and only when it passes back into the state of
water do you actually see anything.
74. When the invisible vapour dissolved in the
air becomes visible, as in mists, clouds, dew, or rain,
it is said to be condensed, 'and this process of
liquefaction is called condensation.
75. The quantity of vapour which the air can
contain varies according to temperature, warm air
being able to hold more than cold air. You can show
AIR.] PHYSICAL GEOGRAPHY. 29
this in a simple way. In breathing you exhale at each
breath a quantity of aqueous vapour ; when the air is
warm, this invisible vapour, as soon as it escapes from
you, mixes with the outer air, and is kept dissolved
there. But if you cool the breath as it leaves your
mouth, the vapour is at once condensed into visible
moisture. Take a mirror, for example, or any other
cold surface, and breathe on it ; the vapour from your
lungs at once shows itself in a film of mist upon the
glass, because the air in contact with the cold surface
is chilled and cannot hold so much vapour, part of
which is condensed. During winter you do not need
a mirror to make the vapour of the breath visible, for
the cold air around you at once condenses this vapour
as it comes from the mouth, and forms the fine cloud
or mist which appears with each breath that you
exhale.
76. As the air is cooled, its power of retaimng
vapour diminishes. When it becomes colder than the
temperature at which it is able to keep its supply of
vapour dissolved, the excess of vapour is condensed
and becomes visible. The temperature at which this
takes place is the point of saturation, or Dew-point
(see Art. 85).
77. Perhaps you may ask how it is that the vapour
so universally present gets into the atmosphere,
and where it comes from. If you pour a little water
into a plate, and set it down in the open air, you
will note, in the course of a day or two, that the water
has sensibly diminished. The air has drunk up part
of it, and will drink up the whole, if the water \z
allowed to stand long enough. What takes place
from a small quantity of water goes on from every
o
o ^ SCIENCE PRIMERS. [the
surface of water on the face of the earth, from every
brook and river and lake, and from the great sea
itself. Water is constantly passing off into vapour
which is received and retained by the air. This pro-
cess is called Evaporation, and the water which
passes off into vapour is said to evaporate.
78. Since warm air can hold more vapour than cold
air, evaporation must be more vigorous in sunshine
than at night, and during summer than during winter.
You have often noticed a great difference in the rate
at which wet roads will dry up. When the sun shines
warmly upon them, an hour or two may be enough to
drive off all the moisture from them, and make them
white and hard again. But if the weather is cold and
dull, they may remain wet and damp for days together.
In the one case the warm air greedily absorbs the
vapour of the water on the roads ; in the other, the
cold air takes up the vapour only in small quantities.
79. Again, on a dry bracing day evaporation goes
on rapidly, because the air has not nearly got all the
quantity of vapour it can hold in solution. On a
damp day, however, when the air contains about as
much vapour as it can hold at that particular tempera-
ture, evaporation is quite feeble, or ceases altogether.
This varying capacity of the air for vapour is the
reason why laundresses find so much difference be-
tween days, in the ease with which they can have
their clothes dried. On some days the air is busy
drinking up vapour everywhere, and then the clothes
dry quickly. Such is especially the case when the
sky is clear and the wind blows, because every moment
a fresh quantity of air comes in contact with the
clothes, carries off some of the vapour, and passes on
AIR.] PHYSICAL GEOGRAPHY. 31
to make way for fresh supplies of thirsty air. On
other days, the air can hardly hold any more vapour ;
and the clothes are found at the end of the day to
be almost as wet as when they were hung out in the
morning.
80. When water evaporates, the vapour carries away
some of the heat of the water with it. Put a drop of
water on the back of your hand, and let it evaporate ;
you notice a sensation of cold, because in evaporating
the vapour has robbed your skin of some of its heat.
This abstracted heat is given out again into the air,
when the vapour is condensed.
81. You see, then, that the air contains invisible
aqueous vapour, which though very small in quantity,
when compared with the amount of nitrogen and
oxygen, is yet enormous when the whole mass of the
atmosphere is considered ; that this vapour rises from
every water-surface over the whole earth by the pro-
cess of evaporation, and that it is brought back again
into the liquid form by the process of condensation.
V. Dew, Mist, Clouds.
82. After sunset, when the sky is clear, you know
that the grass gets wet with dew. In the morning you
may see mists hanging over woods, and streams, and
hills, and gradually melting away as the sun mounts
in the sky. At all times of the year you may watch
how clouds form and dissolve, and form again, ever
changing their size and shape as they move through
the air. Now these are all examples of the conden-
sation of vapour. Let us see how the process takes
place.
^l. Condensation, as we have seen (Art. 76), results
4
32 • SCIENCE PRIMERS. [the
from a cooling of the air. When vapour is condensed,
it does not at once take the form of running water.
The cold glass brought into the warm room has first
a fine film of mist formed upon it, and then by
degrees the clear drops of water come. In reality
mist is made up of exceedingly minute particles of
water, and it is the running together of these which
makes the larger drops. So in nature on the great scale,
when condensation occurs the vapour first appears
as a fine mist. This is always the result of cooling ;
so that, whenever you see a mist or cloud forming,
you may conclude that the air in which it lies is being
cooled.
84. Dew. — This name is given to the wetness
which we notice appearing in the evening or at iiight
upon grass, leaves, or stones, or even sometimes on
our hair. In the morning you have, no doubt, often
watched tlie Httle dewdrops sparkling upon the foliage
and the delicate threads of gossamer. Now this wet-
ness does not come out of the leaves or stones, nor
out of your hair. It is all derived from the air by
condensation, exactly as we saw the film of mist form
upon the cold tumbler in the warm moist air of a
room. In fact, that film of mist was really dew, and
all dew is formed in the same way, and from the same
cause.
85. At night, when the sky is clear, the earth radi-
ates heat rapidly ; that is to say, it gives off into cold
space a great part of the heat which it has received
from the sun during the day (Art. 59). Its surface
consequently becomes cold, as you may have felt
when you put yOur hand upon leaves or stones after
nightfall. The layer of air next the cooled ground is
AIR.] PHYSICAL GEOGRAPHY. 33
chilled below its point of condensation, and the excess
of vapour is deposited as dew upon the grass, twigs,
stones, and other objects. Hence it is that the
temperature at which this condensation begins to
take place is called the Dew-point (Art. 76).
Zd. Mist and Fog. — Another way in which a cold
surface of the earth may produce condensation is
shown by what takes place among mountains. When
a warm moist wind blows upon a chill mountain top,
the air is cooled, and its vapour becomes visible
in the form of a mist or cloud. You can often see
that the cloud is quite solitary, and even shapes itself
to the form of the ground, as if it were a sort of
fleecy cap drawn down over the mountain's head.
This is often well marked in the morning. As day
advances, the ground, warmed by the sun, no longer
cools the air, and hence the mist is gradually re-
absorbed into the atmosphere. But by and by, at the
coming on of night, when the ground is once more
cooled by radiation, if there should be vapour enough
in the air, the mist will re-form, and the mountain
put on his cap again.
87. Cold air, as well as cold ground, condenses the
vapour of warmer air. If you watch what goes on
along the course of a river, you will often see exam-
ples of this kind of condensation. The ground on
either side of the river parts with its heat after
sun-down sooner than the river itself does, and con-
sequently cools the air above it more than the air
above the river is cooled. So when this colder air
from either side moves over to take the place of the
warmer damp air lying on and rising from the river,
condensation ensues in the form of the mist or river-
34 SCIENCE PRIMERS. [the
fog, which so commonly hangs at night and early
morning over streams.
88. Clouds. — It is not on the ground, however,
but up in the air that the chief condensation of vapour
takes place. No feature of everyday occurrence is
more familiar to you than the clouds, which are the
result of this condensation. A cloud is merely a mist
formed by the cooling of warm moist air when it
loses its heat from any cause, such as expansion during
ascent, or contact with currents of cooler air. If you
watch what goes on in the sky, you may often see
clouds in the act of forming. At first a little flake
of white appears. By degrees this grows larger, and
other cloudlets arise and flock together, until at last
the sky is quite overcast with heavy clouds, and rain
begins to fall. The vapour which is thus condensed
in the air has all been obtained by the evaporation
of the water on the earth's surface. It rises with the
warm air, which losing its heat as it ascends, and
coming too in contact with colder layers of the
atmosphere, cannot hold all its vapour, and is obliged
to get rid of the excess, which then condenses into
cloud.
89. On a summer morning the sky is often free
from cloud. As the day advances, and the earth gets
warmed, more vapour is raised ; and as this vapour,
borne upward by the ascending air-currents, reaches
the higher and colder parts of the atmosphere, it is
chilled into the white fleecy clouds which you see
forming about midday and in the afternoon. Towards
evening, when less evaporation takes place, the clouds
cease to grow, and gradually lessen in size until at
night the sky is quite clear. They have been dis-
AIR.] PHYSICAL GEOGRAPHY. 35
solved again by descending and coming in contact
with the warm air nearest to the earth. Again, you
have often noticed that clouds move across the sky.
They are driven along by upper currents of air, and
of course the stronger these currents are the faster do
the clouds travel. In this way the sky is sometimes
completely overcast with clouds which have come
from a distance. By watching these comings and
goings of the clouds, you see how the state of the
vapour in the atmosphere continually changes. At
one time it is condensed into clouds, at another
time evaporated and made invisible by the varying
currents of the air.
VI. Where Rain and Snow come from.
90. You have now traced the vapour which the
sun's heat raises from the rivers, lakes, and seas
of the earth, and you have found it to be condensed
again into visible form in the clouds. But the clouds
do not remain always suspended in the sky. Some-
times they melt away again, and are dissolved into
invisible vapour. But they often disappear in another
way. They let their moisture fall through the air to
the earth, and thus give rise to rain and snow.
91. Rain. — You are well aware that rain always
comes from clouds in the sky. When the sky is clear
overhead, no rain falls. Only when it gets overcast
does the rain come. You c; n watch a dark rain-
cloud gather itself together and discharge a heavy
shower upon the earth. In the illustration of the
cold glass brought into the warm room (Art. 71),
you remember that the film of mist formed upon the
glass was found by degrees to gather into drops,
36 SCIENCE PRIMERS. [the
which trickled down the cold surface. Now the mist
on the glass and the cloud in the sky are both formed
of minute particles of water separated by air. It is
the running together of these particles which gives rise
to the drops. In the one case, the drops run down
the cold glass. In the other case, they fall as drops
of rain through the air. Rain therefore is thus a
further stage in the condensation of the aqueous
vapour of the atmosphere. The minute particles of
the cloud, as condensation proceeds, gather more
moisture round them, until at last they form drops of
water too heavy to hang any longer suspended in the
air. These then fall to the earth as rain- drops.
92. Snow. — But there is another important form
in which the moisture of the clouds may descend to
the surface of the earth. When the weather is cold
enough, there fall to the ground not drops of rain, but
flakes of snow.
93. If you bring snow indoors, it soon melts into
water. If you expose this water for a time it evaporates.
Snow, water, and aqueous vapour are thus only different
forms of the same substance. We say that water can
exist in three forms, — the gaseous, the liquid, and the
solid. Snow is an example of the soHd condition.
94. On a frosty night pools of water are covered
with a hard transparent crust of what is called Ice.
You may break this crust into pieces, but if the cold
continues, a new crust will soon be formed with bits
of the old one firmly cemented in it. And the greater
the cold the thicker will the crust be, until perhaps
the whole of the water in the pools may become solid.
If you take a piece of this solid substance, you find
it to be cold, brittle, and transparent. Brought into
AIR.] PHYSICAL GEOGRAPHY. 37
a warm room it soon melts into water, and you may-
drive off the water as before into vapour. Ice is the
general name given to water when it is in the solid
state, such forms as snow and hail being only difterent
appearances which ice puts on. Whenever water be-
comes colder than a certain temperature it passes into
ice, or freezes, and this temperature is consequently
known as the freezing-point (Physics Primer,
Art. 51).
95. You might suppose that ice is but a shapeless
thing. But gather a few snowflakes, and, that they may
not melt, examine them out of doors. When they lie
Fig. 4. — Forms of Snowflakes.
together in a mass they have a pure opaque whiteness,
but in reality they are as transparent as water ; and it
is only from the way in which they scatter the light
from their many glistening points, that they appear
white. To assure yourselves of this fact, carefully
separate one or two of the flakes upon some dark
surface (the sleeve of a coat will do well), and you
will find that each flake is a more or less perfect star
with six rays, formed of little needles or crystals of
pure transparent ice. The flakes are so delicate that
in falling through the air they are apt to be damaged
by coming against each other. Some of their varieties
are shown in Fig. 4.
96. The upper layers of the atmosphere are much
38 SCIENCE PRIMERS. [the air.
colder than the freezmg-point of water. In the con-
densation which takes place there, the clouds do not
resolve themselves into rain. The vapour of the up-
streaming currents of warm air from the earth's surface
is condensed and frozen in these high regions, and
passes into little crystals, which unite into flakes of
snow. Even in summer the fine white cloudlets which
you see floating at great heights are probably formed
of snow. But in those countries, such as ours, where
in winter the air even at the surface is sometimes very
cold, the snow falls to the ground, and lies there as a
white covering, until returning warmth melts it away.
97. Besides rain and snow, the moisture of the air
takes sometimes the form of Hail, which consists of
little lumps of ice like frozen rain ; and of Sleet, which
is partially melted snow. But rain and snow are the
most important, and it is these two forms which we
must follow a little further.
98. Summary. — Before doing so, let us gather to-
gether the sum of what has been said about the aqueous
vapour of the air. We have learnt that, as every
sheet of water on the face of the globe evaporates, the
air is full of vapour ; that this vapour is condensed into
visible form, and appears as dew, mist, and cloud.
We have learnt further, that the vapour of which
clouds are formed is resolved into rain and snow,
and, in one or other of these forms, descends to the
earth again. There is thus a circulation of water
between the solid earth beneath and the air above.
This circulation is as essential to the earth in making
it a fit habitation for living things, as the circulation of
blood is in keeping our bodies alive. It mixes and
WATER.] PHYSICAL GEOGRAPHY. 39
washes the air, clearing away impurities, such as those
which rise from the chimneys of a town. It moistens
and quickens the soil, which it renders capable of sup-
porting vegetation. It supplies springs, brooks, and
rivers. In short, it is the very mainspring of all the
life of the globe. So important a part of the machinery
of the world deserves our careful consideration. Let
us next attend, therefore, to what becomes of the rain
and the snow after they have been discharged from
the air upon the surface of the earth.
THE CIRCULATION OF WATER ON THE
LAND.
I. W^hat becomes of the Rain.
99. Although air is continually evaporating water
from the surface of the earth, and continually restoring
it again by condensation, yet, on the whole and in
the course of years, there seems to be no sensible
gain or loss of water in our seas, lakes, and rivers ;
so that the two processes of evaporation and con-
densation balance each other.
100. It is evident, however, that the moisture pre-
cipitated at any moment from the air is not at once
evaporated again. When a shower of rain falls, the
roads are not dry the moment the shower is over.
And when heavy rain continues for hours together,
the whole country round may be flooded, and will,
perhaps, remain so for days after the rain has ceased.
The disappearance of the water is due in part to
evaporation, but only in part. A great deal of it goes
out of sight in other ways.
10 1. The rain which falls upon the sea is the
40 SCIENCE PRIMERS. [circulation
largest part of the whole rainfall of the globe, because
the surface of the sea is about three times greater than
that of the land. All this rain gradually mingles with
the salt water, and can then be no longer recognized.
It thus helps to make up for the loss which the sea is
always suffering by evaporation. For the sea is the
great evaporating surface whence most of the vapour
of the atmosphere is derived.
1 02. On the other hand, the total amount of rain
which falls upon all the land of the globe must be
enormous. It has been estimated, for example, that
about 68 cubic miles of water annually descend as rain
even upon the surface of the British Isles, and there
are many much more rainy regions than ours. If you
inquire about this rain which falls upon the land, you
will find that it does not at once disappear, but
begins another kind of circulation. Watch what
happens durmg a shower of rain. If the shower is
heavy, you will notice little runnels of muddy water
coursing down the streets or roads, or. flowing out of
the ridges of the fields. Follow one of the runnels.
It leads into some drain or brook, that into some
larger stream, the stream into a river ; and the river, if
you follow it far enough, will bring you to the sea.
Now think of all the brooks and rivers of the world,
where this kind of transport of water is going on, and
you will at once see how vast must be the part of the
rain which flows off the land into the ocean.
103. But does the whole of the rain flow ofl" at once
into the sea in this way ? Assuredly not, as you can
very easily prove. Suppose that before the rain came
the ground had been very dry, and that after the
shower you dig up a spadeful of earth. Do you find
OF WATER.] PHYSICAL GEOGRAPHY. 41
the ground dry now ? No ', because some of the rain
has soaked into the earth. And if you could dig deep
enough, or if you were to notice what goes on when
workmen are making a deep excavation, you would
find that the ground underneath is not merely damp,
but that it contains plenty of water, and that you could
collect this water, and bring it up to the surface.
Clearly, then, a good deal of the rain which falls
upon the land must sink underground and gather there.
You may think that surely the water which disappears
in that way must be finally withdrawn from the general
circulation which we have been tracing. When it
sinks below the surface, how can it ever get up to the
surface again ?
104. Yet, if you consider for a little, you will be
convinced that whatever becomes of it underneath it
cannot be lost. If all the rain which sinks into the
ground were for ever removed from the surface cir-
culation, you will at once see that the quantity of water
upon the earth's surface must be constantly and visibly
diminishing. The seas must be getting narrower and
shallower; the rivers and lakes must be drying up.
But no such changes, so far as can be seen, are really
taking place. The sea rolls as broadly and deeply as
it has done for many generations past, and the lakes
and rivers remain very much the same. So that if
any of the water which sinks into the earth is never
restored to the surface again, it must be so small a part
as to make no sensible difference on the amount which
is restored. In spite of the rain which disappears into
the ground, the circulation of water between the air,
the land, and the sea continues without perceptible
diminution.
42 SCIENCE PRIMERS. [circulation
105. You are driven to conclude, therefore, that
there must be some means whereby the water under-
ground is brought back to the surface. This is done,
as you will learn in the next section, by Springs,
which gush out of the earth, and bring up water to
feed the Brooks and Rivers, whereby it is borne
into the sea.
106. You can now answer the question, What be-
comes of the Rain ? Most of it sinks into the earth,
and afterwards comes out again in springs ; part of
it is collected into brooks and rivers ; and this part,
in so far as not evaporated, works its way over the
land and falls at last into the sea.
107. Here, then, are two distinct courses which the
rainfall takes — one below ground, and one above. It
will be most convenient to follow the underground
portion first.
II. Ho^v Springs are formed.
108. In this Lesson we are to follow the course of
that part of the rain which sinks below ground. A
little attention to the soils and rocks which form the
surface of a country is enough to show that they differ
greatly from each other in hardness, and in texture or
grain. Some are quite loose and porous, others are
tough and close-grained. They consequently diifer
much in the quantity of water they allow to pass
through them. A bed of sand, for example, is per-
vious ; that is, will let water sink through it freely,
because the little grains of sand lie loosely together,
touching each other only at some points, so as to leave
empty spaces between. The water readily finds its
way among these empty spaces. In fact, the sand-
OF WATER.] PHYSICAL GEOGRAPHY. 43
bed may become' a kind of sponge, quite saturated
with the water which has filtered down from the sur-
face. A bed of clay, on the other hand, is imper-
vious ; it is made up of very small particles fitting
closely to each other, and therefore offering resistance
to the passage of water. Wherever such a bed occurs,
it hinders the free passage of the water, which, unable
to sink through it from above on the way down,
or from below on the way up to the surface again,
is kept in by the clay, and forced to find another line
of escape.
109. Sandy soils are dry because the rain at once
sinks through them ; clay soils are wet because they
retain the water, and prevent it from freely descending
into the earth.
no. When water from rain or melted snow sinks
below the surface into the soil, or into rock, it does
not remain at rest there. If you were to dig a deep
hole in the ground, you would soon find that the water
which Hes between the particles would begin to trickle
out of the sides of your excavation, and gather into
a pool in the bottom. If you baled the water out, it
would still keep oozing from the sides, and the pool
would ere long be filled again. This would show you
that the underground water will readily flow into any
open channel which it can reach.
III. Now the rocks beneath us, besides being in
many cases porous in their texture, such as sandstone,
are all more or less traversed with cracks — sometimes
mere lines, like those of a cracked window-pane, but
sometimes wide and open clefts and tunnels. These
numerous channels serve as passages for the under-
ground water. Hence, although a rock may be so
5
44 SCIENCE PRIMERS. [circulation
hard and close-grained that water does not soak
through it at all, yet if that rock is plentifully supplied
with these cracks, it may allow a large quantity of
water to pass through. Limestone, for example, is a
very hard rock, through the grains of which water can
make but little way; yet it is so full of cracks or
"joints," as they are called, and these joints are often
so wide, that they give passage to a great deal of
water.
112. In hilly districts, where the surface of the
ground has not been brought under the plough, you
will notice that many places are marshy and wet, even
when the weather has long been dry. The soil every-
where around has perhaps been baked quite hard by
the sun ; but these places remain still wet, in spite of
the heat. Whence do they get their water ? Plainly
not directly from the air ; for in that case the rest
of the ground would also be damp. They get it
not from above, but from below. It is oozing out
of the ground ; and it is this constant outcome of
water from below which keeps the ground wet and
marshy. In other places you will observe that the
water does not merely soak through the ground,
but gives rise to a little runnel of clear water. If
you follow such a runnel up to its source, you
will see that it comes gushing out of the ground as
a Spring.
113. Springs are the natural outlets for the under-
ground water. But you ask, why should this water
have any outlets, and what makes it rise to the
surface ?
114. The following diagram (fig. 5) represents the
way in which many rocks lie with regard to each other,
OF WATER.] PHYSICAL GEOGRAPHY.
45
and in which you would meet with them if you were to
cut a long deep trench or section beneath the surface.
They are arranged, as you see, in flat layers or beds.
Let us suppose that <^ is a flat layer of some imper-
vious rock, like clay, and b another layer of a porous
material, like sand. The rain which falls on the sur-
face of the ground, and sinks through the upper bed,
will be arrested by the lower one, and made either to
gather there, or find its escape along the surface of that
Fig. 5. -Origin of Surface Springs.
lower bed. If a hollow or valley should have its bottom
below the level of the line along which the water flows,
springs will gush out along the sides of the valley, as
shown at i" J- in the woodcut. The line of escape may
be either, as in this case, the junction between two
different kinds of rock, or some of the numerous
joints already referred to. Whatever it be, the water
cannot help flowing onward and downward, as long as
there is any passage by which it can find its way ; and
the rocks underneath are so full of cracks, that it has
no difficulty in doing so.
115. But it must happen that a great deal of the
underground water descends far below the level of the
valleys, and even below the level of the sea. And
yet, though it should descend for several miles, it
comes at last to the surface again. To realize clearly
how this takes place, let us follow a particular drop of
water from the time when it sinks into the earth as
46
SCIENCE PRIMERS. [circulation
rain, to the time when, after a long journeying up and
down in the bowels of the earth, it once more reaches
the surface. It soaks through the soil together with
other drops, and joins some feeble trickle, or some
more ample flow of water, which works its way
through crevices and tunnels of the rocks. It sinks in
this way to perhaps a depth of several thousand feet
Fig. 6. — Section of part of a district to show the origin of deep-seated
Springs. The numerous joints in the rocks lead the water down into a main
channel, by which it re-ascends to the surface as a spring at 5.
until it reaches some rock through which it cannot
readily make further way. All this while it has been fol-
lowed by other drops, coursing after it through its wind-
ing passage down to the same barrier at the bottom.
The union of all these drops forms an accumulation
of water, which is continually pressed by what is de-
scending from the surface. Unable to work its way
downward, the pent-up water must try to find escape
in some other direction. By the pressure from above
OF WATER.] PHYSICAL GEOGRAPHY. 47
it is driven through other cracks and passages, winding
up and down until at last it comes to the surface again.
It breaks out there as a gushing spring (see Physics
Primer, Art. 23).
ri6. Thus each of the numerous springs which
issue out of the ground is a proof that there is a cir-
culation of water underneath, as well as upon the
surface of the land. But besides these natural
outlets, other proofs are afforded by the artificial
openings made in the earth. Holes, called Wells,
are actually dug to catch this water. Mines, pits,
quarries, and deep excavations of any kind, are
usually troubled with it, and need to be kept dry
by having it pumped out.
III. The work of Water underground.
117. No form of water seems purer than the clear
crystal spring as it comes bubbling out of the earth.
Water, perfectly pure in a chemical sense, should con-
sist only of the two elements Oxygen and Hydrogen.
But in the water of every spring, no matter how clear
and sparkling it may be, there is something else. If
you take a quantity of perfectly pure water and boil it
down, you may drive the whole of it off in steam, and
not a vestige of anything is left behind. Rain takes
up a little impurity from the air, yet may be regarded
as very nearly pure water. But if you boil down a
quantity of spring water, you find a residue of solid
matter. Sparkling transparency is thus no guide to
the chemical purity of the water (see Chemistry
Primer, Arts. 20, 21).
118. If now rain is water nearly in a state of purity,
and if after journeying up and down underground it
48 SCIENCE PRIMERS. [circulation
comes out again in springs, always more or less mingled
with other materials, it must get these materials from
the rocks through which it travels. They are not
visible to the eye, for they are held in what is called
chemical solution (Chemistry Primer, Art. 23). When
you put a few grains of salt or sugar upon a plate, and
pour water over them, they are dissolved in the water
and disappear. They enter into union with the water.
You cannot see them, but you can still recognize their
presence by the taste which they give to the water
which holds them in solution.
119. So water, sinking from the soil downwards, dis-
solves a little of the substance of the subterranean
rocks, and carries this dissolved material up to the
surface of the ground. But you may say, salt and
sugar are easily acted on by water, hard rocks are
not ; how is it that the springs can get their solid im-
purities from rocks ?
120. You remember that one of the important ingre-
dients in the air is carbonic acid gas, and that this sub-
stance is both abstracted from and supplied to the air
b)^ plants and animals (see Art. 44). In descending
through the atmosphere rain absorbs a little air. As
ingredients of the air, a little carbonic acid gas, particles
of dust and soot, noxious vapours, minute organisms,
and other substances floating in the air, are caught up
by the descending rain, which in this way, as it were,
washes the air, and tends to keep it much more whole-
some than it would otherwise be.
121. But rain not merely picks up impurities from
the air, it gets a large addition when it reaches the
soil. When you take up a little earth from a field
or a garden, you may notice tiny fibres and decaying
OF WATER.] PHYSICAL GEOGRAPHY. 49
roots in it. It contains always more or less organic
matter, and therefore (Art. 44) carbonic and some
other acids. If you put some of the soil on a piece
of iron and thrust it into the fire, you will burn off
the organic matter, remove the carbonic acid, and
change the colour of the soil.
122. Armed with the carbonic acid which it gets
from the air, and with the larger quantity which it
abstracts from the soil, rain-water is prepared to attack
rocks, and to eat into them in a way which pure water
could not do (see Chemistry Primer, Experiment 28).
123. Water containing carbonic acid has a remark-
able effect on many rocks, even on some of the very
hardest. It dissolves more or less of their substance,
and removes it. When it falls for instance on chalk
or limestone, it almost entirely dissolves and carries
away the rock in solution, though still remaining
clear and limpid. In countries where chalk or lime-
stone is an abundant rock, this action of water is
sometimes singularly shown in the way in which
the surface of the ground is worn into hollows. In
such districts, too, the springs are always hard \ that
is, they contain much mineral matter in solution,
whereas rain-water and springs which contain little im-
purity are termed soft (Chemistry Primer, Art. 26).
124. Many of the substances abstracted from below
by the water of springs are useful in the life of plants
and of animals. Lime, salt, and iron, for example, are
all brought up in spring-water, and are all of great
value. Lime furnishes material for the bones of
animals, and iron supplies the colouring matter of
their blood. We obtain, indeed, most of what we
need of these materials from our solid food; yet spring-
50
SCIENCE PRIMERS. [circulation
water, in so far as it contains them, is healthier for
drinking and cooking than rain-water would be.
125. As every spring throughout the world is busy
bringing up materials of some kind to the surface,
Fig 7. — Subterranean Channel dissolved out of Limestone-rock by Water.
it is plain that the amount of rock dissolved and
removed must in the end be very great. You can
now see how there should be open channels and
OF WATER.] PHYSICAL GEOGRAPHY. 51
tunnels for the water underground, for the water is
ahvays eating away a Httle of the surface over which
it flows, thereby widening the cracks and crevices,
and converting them by degrees into wider passages.
In this way large caverns many feet high and many
miles long have been formed underneath the surface
in different parts of the world.
IV. How the surface of the Earth crumbles
away.
126. When a stone building has stood for a few
hundred years, the smoothly dressed face which its
walls received from the mason is usually gone. The
stones are worn into holes and furrows, the carvings
over window and doorway are so wasted that perhaps
you cannot make out what they were meant to repre-
sent. This time-eaten character of old masonry is so
familiar that one always looks for it in an old building,
and when it is absent he at once doubts whether the
building can really be old.
127. Again, in the burying-ground surrounding a
venerable church you see the tombstones more and
more mouldered the older they are. Sometimes,
especially in towns, the inscriptions dating from more
than a few generations back are so greatly wasted
that you cannot now tell whose names and virtues
they were set up to commemorate.
128. This crumbling away of hard stone with the
lapse of time is a common familiar fact to you. lUit
have you ever wondered why it should be so ? What
makes the stone decay, and what purpose is served by
the process ?
129. In the case of buildings and other works of
52 . SCIENCE PRIMERS. [circulation
human construction the decay can be noted and
measured, for the stones, rough and worn as they
may be now, left the hands of the masons with
smoothly dressed surfaces. But the decay is not
confined to human erections. On the contrary, it
goes on over the whole face of the world.
130. It may seem so strange to you to be told that
the surface of the earth is crumbling away that you
should take every opportunity of verifying the state-
ment. Examine all the old buildings and pieces of.
sculpture within your reach. Look at the cliffs and
ravines, the crags and watercourses, in your neigh-
bourhood. At the base of each cliff you will pro-
bably find the ground cumbered with blocks and
heaps of lesser fragments which have fallen from
the rocks above, and after a frosty winter you may
even find the fresh scar whence a new mass has been
detached to add to the pile of ruins below.
131. After examining your own district in this way,
you will, no doubt, find proofs that, in spite of their
apparent steadfastness, even the hardest stones are
really crumbling down. In short, wherever rocks are
exposed to the air they are liable to decay. Now let
us see how this change is brought about.
132. First of all we must return for a moment to the
action of carbonic acid, which has been already
(Art. 123) described. You remember that rain-water
abstracts a little carbonic acid from the air, and that,
when it sinks under the earth, it is enabled by means
of the acid to eat away some parts of the rocks
beneath. The same action takes place with the rain,
which rests upon or flows over the surface of the
ground. The rain-water dissolves out little by little
OF WATER.] PHYSICAL GEOGRAPHY. 53
such portions of the rocks as it can remove. In the
case of some rocks, such as Hmestone, the whole, or
almost the whole, of the substance of the rock is
carried away in solution. In other kinds, the portion
dissolved is the cementing material whereby the mass
of the rock was bound together ; so that when it is
taken away, the rock crumbles into mere earth or
sand, which is readily washed away by the rain.
Hence one of the causes of the mouldering of stone
is the action of the carbonic acid taken up by rain.
133. In the second place, the oxygen of the por-
tion of air contained in rain-water helps to decompose
rocks. " When a piece of iron has been exposed for a
time to the weather, in such a damp climate as that of
Britain, it rusts. You know how, in the course of years,
iron railings get quite eaten through, and how you can
scrape the dirty yellow crust or powder from the cor-
roded surfaces. This rust is a compound substance,
formed by the union of oxygen with iron. It con-
tinues to be formed as long as any of the unrusted
iron remains, since as each crust of rust is washed oft
a new layer of iron is laid open to the attacks of the
oxygen. What happens to an iron railing or a steel
knife, happens also, though not so quickly nor so
strongly, to many rocks. They, too, rust by absorb-
ing oxygen. A crust of corroded rock forms on their
surface, and, when it is knocked off by the rain, a
fresh layer of rock is reached by the ever-present and
active oxygen.
134. In the third place, the surface of many parts
of the world is made to crumble down by means of
frost. You are, no doubt, acquainted with some ot
the eft"ects of frost. You have, probably, noticed that
54 SCIENCE PRIMERS. [circulation
sometimes during winter, when the cold gets very
keen, pipes full of water burst, and jugs filled with
water are cracked from top to bottom. The reason
of this lies in the fact that water expands in freezing.
Ice requires more space than the water would do if it
remained fluid. When ice forms within a confined
space, it exerts a great pressure on the sides of the
vessel, or cavity, which contains it. If these sides are
not strong enough to bear the strain to which they are
put, they must yield, and therefore they crack (see
Physics Primer, Art. 6i).
135. You have now learnt how easily rain finds its
way through soil. Even the hardest rocks are more or
less porous, and take in some water. Hence, when
winter comes, the ground is full of moisture ; not in
the soil merely, but in the rocks. And so, as frost
sets in, this pervading moisture freezes. Now, pre-
cisely the same kind of action takes place with each
particle of water, as in the case of the burst water-
pipe or the cracked jar. It does not matter whether
the water is collected into some hole or crevice,
or is diffused between the grains of the rocks and
the soil. When it freezes it expands, and in so doing
tries to push asunder the walls between which it is
confined.
136. Hence arise some curious and interesting
effects of frost upon the ground. If you walk along
a road just after frost, you see that the small stones
have been partly pushed out of their beds, and that
the surface of the road is now a layer of fine mud.
The frost has separated the grains of sand and clay,
as if they had been pounded down in a mortar.
Hence frost is of great service to the farmer in break-
OF WATER.] PHYSICAL GEOGRAPHY. 55
ing up the soil, and opening it out for the roots and
fibres of plants. When a surface of rock has been
well soaked with rain, and is then exposed to frost,
the grains of the rock undergo the same kind of pres-
sure from the freezing of the water in the pores
between them. They are not so loose and open, how-
ever, as those of the soil are, and they withstand the
action of the frost much better. Of course, the most
porous rocks, or those which hold most water, are
most liable to the effects of this action. Porous
rocks, such as sandstone, are often liable to rapid
decav from frost. The stone has crust after crust
peeled off from it, or its grains are loosened from
each other and washed away by rain.
137. Again, water freezes not only between the com-
ponent grains, but in the numerous crevices or joints,
as they are called, by which rocks are traversed. You
have, perhaps, noticed that on the face of a cliff, or in
a quarry, the rock is cut through by lines running
more or less in an upright direction, and that by
means of these lines the rock is split up by nature,
and can be divided by the quarryman into large four-
sided blocks or pillars. These lines, or joints, have
been already (Art. in) referred to as passages for
water in descending from the surface. You can under-
stand that only a very little water may be admitted
at a time into a joint. But by degrees the joint widens
a little, and allows more water to enter. Every time
the water freezes it tries hard to push asunder the two
sides of the joint. After many winters, it is at last
able to separate them a little ; then more water enters,
and more force is exerted in freezing, until at last the
block of rock traversed by the joint is completely split
6
56
SCIENCE PRIMERS. [circulation
up. When this takes place along the face of a cliff,
one of the loosened parts may fall off and actually roll
down to the bottom of the precipice.
138. This kind of waste is represented in the ac-
companying woodcut (Fig. 8), which gives a section of
Fig. 8 —Waste of a Clift.
a cliff wherein the rocks are traversed by perpendicular
joints. These have been, widened along the front
until large blocks have been wedged off and have
OF WATER.] PHYSICAL GEOGRAPHY. 57
fallen to the ground. In countries exposed to severe
winters, the waste caused by frosts along lines of steep
cliff is often enormous.
139. In addition to carbonic acid, oxygen, and
frost, there are still other influences at work by which
the surface of the earth is made to crumble. For ex-
ample, when, during the day, rocks are highly heated
by strong sunshine, and then during night are rapidly
cooled by radiation, the alternate expansion and con-
traction caused by the extremes of temperature loosen
the particles of the stone, causing them to crumble
away, or even making successive crusts of the stone
fall off.
140. Again; rocks which are at one time well
soaked with rain, and at another time are liable to be
dried by the sun's rays and by wind, are apt to
crumble away.
141. And thus you see that from a variety of causes
the solid rocks of the earth are liable to continual decay
and removal. The hardest stone, as well as the softest,
must yield in the end, and moulder down. They do
not all indeed decay at the same rate. If you look
more narrowly at the wall of an ancient building, you
w411 see almost every variety in the degree of decay.
Some of the stones are hardly worn at all, while others
are almost wholly gone. As this takes place in a build-
ing, you may be sure it must take place also in nature,
and that cliffs or crags formed of one kind of stone
will crumble down faster than others, and will do so
in a different kind of way.
142. If then it be true, as it is, that a general
wasting of the surface of the land goes on, you may
naturally ask why this should be. The world seems
58 SCIENCE PRIMERS. [circulation
SO fair and beautiful, that you cannot perhaps realize
to yourselves that there should be so much decay on
its surface. You may be even inclined at first to con-
sider the decay as a misfortune hardly to be ex,plained.
But instead of being a misfortune, the mouldering of
the surface is in reality necessary to make the earth
fit to be the dwelling-place of plants and animals.
To it we owe the scooping out of valleys, and ravines,
and the picturesque outlines of crags and hills. Out
of the crumbled stones all soil is made, and on the
formation and renewal of the soil we depend for our
daily food. How this is brought about will be told
in the next Lesson.
V. What becomes of the crumbled parts
of Rocks. How Soil is made.
143. Take up a handful of soil from any field or
garden, and look at it attentively. What is it made
of? You see little pieces of crumbling stone, particles
of sand and clay, perhaps a few vegetable fibres ;
and the whole soil has a dark colour from the
decayed remams of plants and animals diffused
through it. Now let us in the present Lesson try to
learn how these different materials have been brought
together.
144. We return again to the general mouldering of
the surface of the land. The words " decay," " waste,"
and others of similar meaning, are applied to this pro-
cess. But in reality, although the rocks may crumble
away, and thereby grow less in size year by year, there
is no actual loss of material to the surface of the earth.
The substance of the rock may decay, but it is not
destroyed. It only changes its condition and its form.
OF WATER.] PHYSICAL GEOGRAPHY. 59
What, then, becomes of all this material which is con-
tinually being worn from the rocks around us ?
145. Every drop of rain which falls upon the land
helps to alter the surface. You have followed the
chemical action of rain when it dissolves parts of
rocks. It is by the constant repetition of the process,
drop after drop, and shower after shower, for years
together, that the rocks become so wasted and worn.
But the rain has also a mechanical action.
146. Watch what happens when the first pattering
drops of a shower begin to fall upon a smooth surface
of sand, such as that of a beach. Each drop makes
a little dint or impression. It thus forces aside the
grains of sand. On sloping ground, whe^e the drops
Fig. 9. — Prints impressed on Clay or Sand by Drops of Rain.
can run together and flow downward, they are able
to push or carry the particles of sand or clay along.
This is called a mechanical action ; while the actual
solution of the particles, as you would dissolve sugar
or salt, is a chemical action. Each drop of rain may
act in either or both of these ways.
147. Now you will readily see how it is that rain
does so much in the destruction of rocks. It not only
dissolves out some parts of them, and leaves a crum-
bling crust on the surface, but it washes away this
crust, and thereby exposes a fresh surface to decay.
6o SCIENCE PRIMERS. [circulation
There is in this way a continual pushing along of
powdered stone over the earth's surface. Part of this
material accumulates in hollows, and on sloping or
level ground ; part is swept into the rivers, and carried
away into the sea.
148. It is this crumbled stone of which all our soils
are made, mingled with the remains of plants and
animals. Soils differ, therefore, according to the kind
of rock out of which they have been formed. Sand-
stone, for example, will give rise to a sandy soil ;
limestone to a limy or calcareous soil j clay-rocks to a
clayey soil.
149. But for this crumbling of the rocks into soil,
the land would not be covered with verdure as it is.
Bare sheets of undecaying stone would give no foot-
ing for the roots of plants. But by the decay of their
surface, they get covered with fertile soil, all over the
valleys and plains ; and only where, as in steep banks
and cliffs, they rise too abruptly to let their crumbled
remains gather round them, do they stand up naked
and verdureless.
150. As the mouldering of the surface of the land is
always going on, there is a constant formation of soil.
Indeed, if this were not the case, if after a layer of
soil had been formed upon the ground, it were to
remain there unmoved and unrenewed, the plants
would by degrees take out of it all the earthy materials
they could, and leave it in a barren or exhausted
state. But some of it is being slowly carried away
by rain, fresh particles from mouldering rocks are
washed over it by the same agent, while the rock or
sub-soil underneath is all the while decaying into
soil. The loose stones, too, are continually crumbling
OF WATER.] PHYSICAL GEOGRAPHY. 6i
down and making new earth. And thus, day by day,
the soil is slowly renewed.
151. Plants, also, help to form and renew the soil.
They send their roots among the grains and joints of
the stones, and loosen them. Their decaying fibres
supply most of the carbonic acid by which these stones
are attacked, and furnish also most of the organic
matter in the soil. Even the common worms, which
you see when you dig up a spadeful of earth, are of
great service in mixing the soil and bringing what lies
underneath up to the surface.
152. When we think about this decay and renewal
of soil, we see that in reahty the whole surface of the
land may be looked upon as travelling downward or
seaward. The particles worn from the sides and
crests of the high mountains may take hundreds or
thousands of years on the journey ; they may lie for
a long time on the slopes ; they may then be swept
down and form part of the soil of the valleys ; thence
they may be in after years borne away and laid down
on the bed or bank of a river ; and thus, after many
halts by the way, they at last reach the sea.
153. In order to form some idea of the extent to
which the surface of the land is cleared of its loose
soil by rain, you should notice what takes place even
in this country after every series of heavy showers.
Each little runnel and brook becomes muddy and
discoloured from the quantity of soil, that is, decayed
rock, which is washed into it by the rain from the
neighbouring slopes. The mud which darkens the
water is made of the finer particles of the decomposed
rocks ; the coarser parts are moving along at the bottom
of the water. When you watch these streamlets at
62 SCIENCE PRIMERS. [circulation
their work, and when you remember that what they
are doing now they have been doing for ages past, you
will understand how greatly the surface of a country
may come to be changed by the action of what at
first seems so insignificant a thing as Rain.
VI. Brooks and Rivers. Their 'Origin.
154. We must now go back to an earlier Lesson
(Art. 107), where the way in which rain is disposed
of was referred to. You remember that one part of
the rain sinks under the ground, and you have traced
its progress there until it comes to the surface again.
You have now to trace, in a similar way, the other
portion of the rainfall which flows along the surface in
brooks and rivers.
155. You cannot readily meet with a better illus-
tration of this subject than that which is furnished by
a gently sloping road during a heavy shower of rain.
Let us suppose that you know such a road, and that
just as the rain is beginning you take up your station
at some part where the road has a well-marked descent.
At first you notice that each of the large heavy drops
of rain makes in the dust, or sand, one of the little
dints or rain-prints already described (Art. 146). As
the shower gets heavier these rain-prints are effaced,
and the road soon streams with water. Now mark in
what manner the water moves.
156. Looking at the road more narrowly, you re-
mark that it is full of little roughnesses — at one place
a long rut, at another a projecting stone, with many
more inequalities which your eye could not easily
detect when the road was dry, but which the water at
once discloses. Every little dimple and projection
OF WATER.] PHYSICAL GEOGRAPHY. 63
affects the flow of the water. You see how the
raindrops gather together into slender streamlets of
running water which course along the hollows, and
how the jutting stones and pieces of earth seem to
turn these streamlets now to one side and now to
another.
157. Towards the top of the slope only feeble
runnels of water are to be seen. But further down
they become fewer in number, and at the same time
larger in size. They unite as they descend ; and the
larger and swifter streamlets at the foot of the descent
are thus made up of a great many smaller ones from
the higher parts of the slope.
158. Now this sloping roadway, with its branching
rills of rain, coursing down the slope, and uniting into
larger streams as they advance, shows very well the
way in which the rain runs off the sloping surface of a
country or a continent, and we shall return to the
illustration again.
159. Why does the water run down the sloping road ?
why do rivers flow ? and why should they always move
constantly in the same direction ? They do so for the
same reason that a stone falls to the ground when it
drops out of your hand ; because they are under the
sway of that attraction towards the centre of the eartli,
to which, as you know, the name of Gravity (Physics
Primer, Art. 4) is given. Every drop of rain falls to
the earth because it is drawn downwards by the force
of this attraction. When it reaches the ground it is
still, as much as ever, under the same influence ; and it
flows downwards in the readiest channel it can find.
Its fall from the clouds to the earth is direct and
rapid ; its descent from the mountains to the sea, as
64 SCIENCE PRIMERS. [circulation
part of a stream, is often long and slow ; but the cause
of the movement is the same in either case. The
winding to and fro of streams, the rush of rapids, the
roar of cataracts, the noiseless flow of the deep sullen
currents, are all proofs how paramount is the sway of
the law of gravity over the waters of the globe.
1 60. Drawn down in this way by the action of
gravity, all that portion of the rain which does not
sink into the earth must at once begin to move down-
wards along the nearest slopes, and continue flowing
until it can get no further. On the surface of the land
there are hollows called Lakes, which arrest part of
the flowing water, just as there are hollows on the
road which serve to collect some of the rain. But in
most cases they let the water run out at the lower end
as fast as it runs in at the upper, and therefore do not
serve as permanent resting places for the water. The
streams which escape from lakes go on as before, work-
ing their way to the sea-shore. So that the course of
all streams is a downward one ; and the sea is the
great reservoir into which the water of the land is
continually pouring.
161. If the surface of a country were a mere long
smooth ridge, like the roof of a house, the rain would
quickly flow down on either side into the sea. But this
is by no means the general character of the surface of
the land. Mountains, hills, valleys, gorges, and lakes
give a most uneven and varied outline. But besides
these greater inequalities which strike the eye at once,
even places which seem at first quite level have usually
some slope or some slight unevennesses ; just as on the
road you found that there may be many little irregu-
larities of surface, which you would not notice until
OF WATER.] PHYSICAL GEOGRAPHY. 65
the rain found them out. Water is thus a most accu-
rate measurer of the levels of a country. It will not
flow up a slope, but always seeks the lowest level it
can find.
162. You can see, then, that though the rain should
fall equally over the whole surface of a country, it can-
not flow equally over that surface, because the ground
is uneven, and the rain runs off into the hollows. It
is this unevenness which makes the rain collect into
brooks, and these into rivers.
163. The brooks and rivers of a country are thus the
natural drains, by which the surplus rainfall, not re-
quired by the soil or by springs, is led back again into
the sea. When we consider the great amount of rain,
and the enormous number of brooks in the higher
parts of the countr}^, it seems, at first, hardly possible
for all these streams to reach the sea without over-
flowing the lower grounds. But this does not take
place ; for when two streams unite into one, they do
not require a channel twice as broad as either of their
single water-courses. On the contrary, such an union
often gives rise to a stream which is not so broad as
either of the two from which it flows. But it becomes
swifter and deeper. In this way thousands of stream-
lets, as they come together in their descent, are made
to take up less and. less room, until the surplus waters
of a whole vast region are borne into the sea by one
single river-channel.
164. Let us return to the illustration of the roadway
in rain. Starting from the foot of the slope, you found
the streamlets of rain getting smaller and smaller, and
when you came to the top there were none at all. If,
however, you were to descend the road on the other
66 SCJEACE PRIMERS. [circulation
side of the ridge, you would probably meet with other
streamlets coursing down-hill in the opposite direction.
At the summit the rain seems to divide, part flomng
oft to one side, and part to the other.
165. In the same way, were you to ascend some
river from the sea, you would watch it becoming
narrower as you' traced it inland, and branching more
and more mto tributaiy streams, and these again
subdividing into almost endless little brooks. But take
any of the branches which unite to form the main
stream, and trace it upward. You come, in the end,
to the first beginnings of a little brook, and going a
little further you reach the summit, down the other
side of which all the streams are flowing to the oppo-
site -quarter. The line which separates two sets of
streams in this way is called the Water- shed. In
England, for example, one series of rivers flows into
the Atlantic, another into the North Sea. If you trace
upon a map a line separating all the upper streams of
the one side from those of the other, that line will
mark the water-shed of the country.
166. But there is one important point where the
illustration of the road in rain quite fails. It is only
when rain is falling, or immediately after a heavy
shower, that the rills are seen upon the road. When
the rain ceases the Avater begins to dr}' up, till in
a short time the road becomes once more firm and
dusty. But the brooks and rivers do not cease to flow
when the rain ceases to fall. In the heat of summer,
when perhaps there has been no rain for many days
together, the rivers still roll on, smaller usually than
they were in winter, but still with ample flow. What
keeps them full? If you remember what you have
OF WATER.] PHYSICAL GEOGRAPHY. 67
»
already been told about underground-water, you will
answer that rivers are fed by springs as v/ell
as by rain.
167. Though the weather may be rainless, the springs
continue to give out their supplies of water, and these
keep the rivers going. But if great drought comes,
many of the springs, particularly the shallow ones,
cease to flow, and the rivers fed by them shrink up or
get dry altogether. This is the case with the rivers of
this countr}^, which are all, comparatively speaking,
very small. The great rivers of the globe, such as the
Mississippi, drain such vast territories, that any mere
local rain or drought makes no sensible difference in
their mass of water.
168. In some parts of the world, however, the rivers
are larger in summer and autumn than they are in
winter and spring. The Rhine, for instance, begins
to rise as the heat of summer increases, and to fall as
the cold of winter comes on. This happens because
the river has its source among snowy mountains.
Snow melts rapidly in summer, and the water which
streams from it finds its way into the brooks and rivers,
wliich are thereby greatly swollen. In winter, on the
other hand, the snow remains unmelted ; the moisture
which falls from the air upon the mountains is chiefly
snow; and the cold is such as to freeze the brooks.
Hence the supplies of water at the sources of these
rivers are, in winter, greatly diminished, and the rivers
themselves become proportionately smaller.
169. Summary. — To sum up what has been stated
in this and the preceding Lessons regarding the circu-
lation of water : — From the highest parts of the land
7
68 SCIENCE PRIMERS. [circulation
■ ■ ■ ■ ■ ■ * ■ — - — ■ — ■
down to the sea, water is continually travelling down-
ward. It does not pour over the whole surface, but
gathers into the hollows, where it forms streams which
wind to and fro, always seeking a lower level, till at
last they lose themselves in the sea. From the sea
vapour is constantly rising into the air, whence it is
brought back and condensed upon the land as rain
or snow, which feeds the streams that flow downward
into the sea. This circulation of water goes on with-
out ceasing.
VII. Brooks and Rivers. Their work.
170. In the first lesson of this little book you were
asked to watch, the doings of a river. Let us now
again return to the same scene, but before the storm
which was then described. The river is not yet swollen
with the sudden and heavy rain. It flows gently over
its pebbly channel, not covering the whole of it, per-
haps, but leaving banks of gravel and pools of water
between which the clear current, much diminished by
drought, winds its way. The river seems to be doing
nothing else than lazily carrying the surplus water of
the land towards the sea. You might be surprised to
be told that it has any work to do, and even now is
doing it.
171. But consider whence the water of the river
comes. We have found that it is largely derived from
springs, and that all spring-water contains more or less
mineral materials dissolved out of the brooks. Every
river, therefore, is carrying not merely water, but large
quantities of mineral matter into the sea. It has been
calculated, for instance, that the Rhine in one year
carries into the North Sea lime enough to make three
OF WATER.] PHYSICAL GEOGRAPHY. 69
hundred and thirty-two thousand millions of oyster
shells. This chemically-dissolved material is not
visible to the eye, and in no way affects the colour of
the water. At all times of the year, as long as the
water flows, this invisible transport of some of the
materials of rocks must be going on.
172. But let us now again watch the same river in
flood. The water is no longer clear, but dull and dirty.
Y5u ascertained that this discoloration arises from
mud and sand suspended in the water. You may stand
for hours and watch the swollen, turbid torrent rolling
down its channel. During that time many tons of
gravel, sand, and mud must be swept past you.
You see that over and above the mineral matter in
chemical solution, the river is hurrying seaward with
vast quantities of other and visible materials. And
thus it is clear that at least one great part of the work
of rivers must be to transport the mouldered parts of
the land which are carried into them by springs or
by rain.
173. But the rivers, too, help in the general destruc-
tion of the surface of the land. Of this you may readily
be assured, by looking at the sides or bed of a stream
when the water is low. Where the stream flows over
hard rock, you find the rock all smoothed and ground
away ; and the stones lying in the water-course are
all more or less rounded and smoothed. When these
stones were originally broken by frosts or otherwise,
from crags and clitfs, they were sharp-edged, as you
can prove by looking at the heaps of blocks lying at
the foot of any precipice, or steep bank of rock. But
when they fell, or were washed into the river, they
began to get rolled and rubbed, until their sharp edges
70
SCIENCE PRIMERS. [circulation
were ground away, and they came to wear the smooth
rounded forms which we see in the ordinary gravel.
174. While the stones are ground down, they, at
the same time, grind down the rocks which form the
Fig. 10. — Potholes excavated by a Stream in the Rocks of its Bed.
sides and bottom of the river-channel over which they
are driven. You can even see in some of the eddies
of the stream how the stones are kept moving round
OF WATER.] PHYSICAL GEOGRAPHY. 71
until they actually excavate deep round cavities, called
pot-holes, in the solid rock. When the water is low,
as during the droughts of summer, some of these cavi-
ties are laid bare, and you may then observe how well
they have been polished. Their general appearance
is shown in Fig. 10.
175. Now, it is clear that two results must follow
from this ceaseless wear and tear of rocks and stones
in the channel of a stream. In the first place, a great
deal of mud and sand must be produced ; and, in the
second place, the bed of the river must be ground
down so as to become deeper and wider. The sand
and mud are added to the other similar materials
washed into the streams by rain from the mouldering
surface of the land. By the deepening and widening
of the water-courses, such picturesque features as
gorges and ravines are excavated out of the solid
rock.
176. You have now seen why the rivers are muddy.
Let us inquire what becomes of all the mud, sand,
gravel, and blocks of stone which they are continually
transporting.
177. Look, again, at the channel of a river in sum-
mer. You see it covered with sheets of gravel in one
place, beds of sand in another, while here and there
a piece of hard rock sticks up through these different
kinds of river-stuff. Note some portion of the loose
materials, and you find it to be continually shifting. A
patch of gravel or sand may remain for a time, but
the little stones and grains of which it is made up
are always changing as the water covers and moves
them. In fact, the loose materials over which the river
flows are somewhat like tlie river itself You come
72 SCIENCE PRIMERS. [circulation
back to its banks after many years, and you find the
river there still, with the same ripples, and eddies, and
gentle murmuring sound. But though the river has
been there constantly all the time, its water has been
changing every minute, as you can watch it changing
still. So, although the channel is always more or less
covered with loose materials, these are not always the
same. They are perpetually being pushed onward,
and others, from higher up the stream, come behind
to take their place.
178. It is not in the bottoms of the rivers, then, that
the material worn away from the surface of the land can
find any lasting rest. And yet the rivers do get rid of
a good deal of this material as they roll along. You
have, perhaps, noticed that a river is often bordered
with a strip of flat plain, the surface of which is only
a few feet above the level of the water. Most of our
rivers have such margins, and, indeed, seem each to
wind to and fro through a long, level, meadow-like
plain. Now this plain is really made up from the finer
particles of the decomposed rocks which the river has
carried along. During floods, the river, swollen and
muddy, rises above its banks, and spreads over the low
ground on either side. Whenever this takes place, the
overflowing water moves more slowly over the flats ;
and, as its current is thus checked, it cannot hold so
much mud and sand, but allows some of these ^naterials
to settle down to the bottom. In this way the over-
flowed tracts get a coating of soil laid over them
by the river, and when the waters retire this coating
adds a little to the height of the plain. The same
thing takes place year after year, until by degrees the
plain gets so far raised that the river, which all this
OF WATER.] PHYSICAL GEOGRAPHY. 73
while is also busy deepening its channel, cannot over-
flow it even at the highest floods. In course of time
the river, as it winds from side to side, cuts away
slices of the plain and forms a newer one at a lower
level. And thus a series of terraces is gradually made,
rising step by step above the river.
Fig. II. — Section of the successive terraces (i,.2, 3! of sand, earth, and
gravel formed by a River along a valley (s— s).
179. Still the laying down of its sand and mud by
a river to form one or more such river-terraces is,
after all, only a temporary disposal of these materials.
They are still liable to be carried away, and in truth
they are carried off continually as the river eats away
its banks.
180. When the current of a river is checked as it
enters the sea or a lake, the feebler flow of the water
allows the sand and mud to sink to the bottom. By
degrees some portions of the bottom come in this
way to be filled up to the surface of the river, and
wide flat marshy spaces are formed on either side of
the main stream. During floods these spaces are
overflowed with muddy water, in the same way as in
the case of the valley plains just described, and a
coating of mud or sand is laid down on them until
they slowlv rise above the ordinary level of the river,
which winds about among them in endless branching
streams. Vegetation springs up on these flat swampy
lands ; animals, too, find food and shelter there ; and
74
SCIENCE PRIMERS. [circulation
thus a new territory is made by the work of the
river.
1 8 1. These flat river-formed tracis are called Deltas,
because the one which was best known to the ancients,
that of the Nile, had the shape of the Greek letter
A {delta). This is the general form which is taken
Fig. 12. — Delta of the Mississippi.
by accumulations at the mouths of rivers ; the flat
delta gets narrow towards the inland, and broader
towards the sea. Some of them are of enormous
size ; the delta of the Mississippi, for example.
182. Each delta, then, is made of materials worn
from the surface of the land, and brought down by the
OF WATER.] PHYSICAL GEOGRAPHY. ' 75
river. And yet vast though some of these deltas are,
they do not show all the materials which have been
so worn away. A great deal is carried far out and
deposited on the sea-bottom ; for the sea is the great
basin into which the spoils of the land are continually
borne.
VIII. Snow-fields and Glaciers.
183. Having now followed the course taken by the
water which falls on the land as rain, we come to that
taken by snow (Art. 92).
184. On the tops of some of the highest mountains
in Britain snow lies for great part of the year. On
some of them, indeed, there are shady clefts wherein
you may meet with deep snow-wreaths even in the
heat of summer. It is only in such cool and sheltered
spots, however, that the snow remains unmelted.
185. But in other parts of Europe, where the moun-
tains are more lofty, the peaks and higher shoulders of
the hills gleam white all the year with unmelted snow.
Hardly anything in the world will impress you so
much as the silence and grandeur of these high snowy
regions. Seen from the valleys, the mountains look so
vast and distant, so white and pure, yet -catching up
so wonderfully all the colours which glow in the sky
at morn or even, that they seem to you at first rather
parts of the heaven above than of the solid earth on
which we live. But it is when you climb up fairly
into their midst thnt their wonderful stateliness comes
full before you. Peaks and pinnacles of the most
dazzling whiteness glisten against the dark blue of the
sky, streaked here and there with lines of purple
shadow, or with knobs of the dark rock projecting
76 SCIENCE PRIMERS. [circulation
through the white mantle which throws far and wide
its heavy folds over ridge and slope, and sends long
tongues of blue ice down to the meadows and vine-
yards of the valleys. There is a deep silence over
this high frozen country. Now and then a gust of
wind brings up from the far distance the sound of
some remote waterfall or the dash of a mountain
torrent. At times, too, there comes a harsh roar as
of thunder, when some mass of ice or snow, loosened
from the rest, shoots down the precipices. But these
noises only make the silence the deeper when they
have passed away.
1 86. Let us see why it is that perpetual snow should
occur in such regions, and what part this snow plays
in the general machinery of the world.
187. You have learnt (Art. 96) that the higher parts
of the atmosphere are extremely cold. You know also
that in the far north and the far south, around those
two opposite parts of the earth's surface called the
Poles, the chuiate is extremely cold— so cold as to give
rise to dreary expanses of ice and snow, where sea and
land are frozen, and where the heat of summer is not
enough to thaw all the ice and drive away all the
snow. Between these two polar tracts of cold, wher-
ever mountains are lofty enough to get into the high
parts of the atmosphere where the temperature is
usually below the freezing-point, the vapour condensed
from the air falls upon them, not as rain, but as snow.
Their heads and upper heights are thus covered with
perpetual snow. In such high mountainous regions
the heat of the summer always melts the snow from,
the lower hills, though it leaves the higher parts still
covered. From year to year it is noticed that there is
OF WATER.] PHYSICAL GEOGRAPHY. 77
a line or limit below which the ground gets freed of
its snow, and above which the snow remains. This
limit is called the snow- line, or the limit of
perpetual snow. Its height varies in different
parts of the world. It is highest in the warmer
regions on either side of the equator, where it reaches
to 15,000 feet above the sea. In the cold polar
tracts, on the other hand,' it approaches the sea-level.
In other words, while in the polar tracts the climate
is so cold that perpetual snow is found even close
to the sea-level, the equatorial regions are so warm
that you must climb many thousand feet before you
can reach the cold layers of the air where snow can
remain all the year.
188, You have no doubt watched a snow-storm.
You have seen how at first a few flakes begin to show
themselves drifting through the air ; how they get
more in number and larger in size, until the ground
begins to grow white ; and how, as hours go on, the
whole country becomes buried under a white pall,
perhaps six inches or more in thickness. You see
one striking difference between rain and snow. If
rain had been falling for the same length of time,
the roads and fields would still have been visible, for
each drop of rain, instead of remaining where it fell,
would either have sunk into the soil, or have flowed
off into the nearest brook. But each snowflake, on
the contrary, lies where it falls, unless it happens to
be caught up and driven on by the wind to some
other spot where it can finally rest. Rain disappears
from the ground as soon as it can ; snow stays still
as long as it can.
189. You will see at once that this marked differ-
78 SCIENCE PRIMERS. [circulation
ence of behaviour must give rise to some equally
strong differences in the further procedure of these
two kinds of moisture. You have followed the pro-
gress of the rain; now let us try to find out what
becomes of the snow.
190. In such a country as ours, where there is
no perpetual snow, you can without much difficulty
answer this question. Each fall of snow in winter-time
remains on the ground as long as the air is not warm
enough to melt it. Evaporation, indeed, goes on from
the surface of snow and ice, as well as from water; so
that a layer of snow would in the end disappear, by
being absorbed into the air as vapour, even though none
of it had previously been melted into running water.
But it is by what we call a thaw that our snow is
chiefly dissipated ; that is, a rise in the temperature,
and a consequent melting of the snow. When the
snow melts, it sinks into the soil and flows off into
brooks in the same way as rain. Its after course
needs not to be followed, for it is the same as that
of rain. You will only bear in mind that if a
heavy fall of snow should be. quickly thawed, then
a large quantity of water will be let loose over the
country, and the brooks and rivers will rise rapidly
in flood. Great destruction may thus be caused by
the sudden rise of rivers and the overflowing of their
banks.
191. In the regions of perpetual snow the heat of
summer cannot melt all the snow which falls there in
the year. What other way of escape, then, can the
frozen moisture find ? That it must have some means
of taking itself off the mountains is clear enough ; for
if it had not, and if it were to accumulate there from
OF WATER.] PHYSICAL GEOGRAPHY. 79
year to year and from century to centur}-, then the
mountains would grow into vast masses of snow, reach-
ing far into the sky, and spreading out on all sides,
so as to bury by degrees the low lands around. But
nothing of this kind takes place. These solemn
snowy heights wear the same unchanged look from
generation to generation. There is no bur}dng of
their well-known features under a constantly increas-
ing depth of snow.
192. You will remember that the surplus rainfall
flows off by means of rivers. Now the surplus
snow-fall above the snow-line has a similar kind of
drainage. It flows off by means of what are called
Glaciers.
193. When a considerable depth of snow has accu-
mulated, the pressure upon the lower layers from
what lies above them squeezes them into a firm
mass. The surface of the ground is usually sloped
in some direction, seldom quite flat. And among
the high mountains the slopes are often, as you
know, very steep. When snow gathers deeply on
sloping ground, there comes a time when the force
of gravity overcomes the tendency of the pressed
snow to remain where it is, and then the snow-
begins to slide slowly down the slope. From one
slope it passes on downwards to the next, joined
continually by other sliding masses from neighbour-
ing slopes until they all unite into one long tongue
which creeps slowly down some valley to a point
where it melts. This tongue from the snow-fields is
the glacier. It really drains these snow-fields of their
excess of snow as much as a river drains a district of
its excess of water.
8
8o SCIENCE PRIMERS. [circulation
194. But the glacier which comes out of the snow-
fields is itself made not of snow, but of ice. The
snow, as it slides downward, is pressed together into
ice. You have learned that each snowflake is made
of little crystals of ice. A mass of snow is thus only
a mass of minute crystals of ice with air between.
Hence when the snow gets pressed together, the air
is squeezed out, and the separated crystals of ice
freeze together into a solid mass. You know that
you can make a snowball very hard by squeezing it
firmly between the hands. The more tightly you
press it the harder it gets. You are doing to it just
what happens when a glacier is formed out of the
eternal snows. You are pressing out the air, and
allowing the little particles of ice to freeze to each
other and form a compact piece of ice. But you
cannot squeeze nearly all the air out, consequently
the ball, even after all your efforts, is still white from
the imprisoned air. Among the snowfields, however,
the pressure is immensely greater than yours ; the air
is more and more pressed out, and at last the snow
becomes clear transparent ice.
195. A glacier, then, is a river, not of water, but of
ice, coming down from the snow-fields. It descends
sometimes a long way below the snow-line, creeping
down very slowly along the valley which it covers
from side to side. Its surface all the tim.e is melting
during the day in summer, and streams of clear
water are gushing along the ice, though, when night
comes, these streams freeze. At last it reaches some
point in the valley beyond which it cannot go, for
the warmth of the air there is melting the ice as fast
as it advances. So the glacier ends, and from its
OF WATER.] PHYSICAL GEOGRAPHY.
8i
melting extremity streams of muddy water unite into
a foaming river, which bears down the drainage of
the snow-fields above.
196. In the accompanying woodcut (fig. 13) some of
the chief characters of a glacier are shown. In the dis-
tance rise the snowy heights, among which the snow-
FiG. 13. — View of a Glacier, with its Moraines, Perched Blocks of rock, ice-
worn Bosses of rock and escaping River.
fields lie. From either side the snow is drained oft"
into the main valley, where it forms the glacier, which
winds with all the windino:s of the vallev till it ends
abruptly, as you see, and a river rushes out from the
melting end of the ice.
197. A river wears down the sides and bottom of
its channel, and thus digs out a bed for itself in even
82 SCIENCE PRIMERS. [circulation
the hardest rock, as well as in the softest soil
(Art. 173). It sweeps down, too, a vast quantity of
mud, sand, and stones from the land to the sea
(Art. 172). A glacier performs the same kind of
work, but in a very diiferent way.
198. When stones fall into a river they sink to the
bottom, and are pushed along there by the current.
When mud enters a river it remains suspended in the
water, and is thus carried along. But the ice of a
glacier is a solid substance. Stones and mud which
fall upon its surface remain there, and are borne
onward with the whole mass of the moving glacier.
They form long lines of rubbish upon the glacier, as
shown in fig. 13, and are called moraines. Still
the ice often gets broken up into deep cracks, opening
into yawning clefts or crevasses, which sometimes
receive a good deal of the earth and stones let loose
by frost or otherwise from the sides of the valley. In
this way loose materials fall to the bottom of the ice,
and reach the sohd floor of the vallev down which the
ice is moving; while at the same time similar rubbish
tumbles between the edge of the glacier and the side
of the valley.
199. The stones and grains ofsand which get jammed
between the ice and the rock over w^hich it is moving
are made to score and scratch this rock. They form
a kind of rough polishing powder, whereby the glacier
is continually grinding down the bottom and sides of
its channel. If you creep in below the ice, or catch a
sight of some part of the side from which the ice has
retired a little, you will -find the surface of the rock all
rubbed away and covered with long scratches made
by the sharp points of the stones and sand. Some of
OF WATER.] PHYSICAL GEOGRAPHY.
83
the rounded ice-worn bosses of rock are shown in the
fore-ground of the diagram (fig. 13).
200. You will now see the reason why the river,
which escapes from the end of a glacier, is always
muddy. The bottom of the glacier is stuck all over
w^th stones, which are scraping and wearing down the
rock underneath. A great deal of fine mud is thus
produced, which, carried along by streams of water
flowing in channels under the glacier, emerges at the
far end in the discoloured torrents which there sweep
from under the ice.
Fig. 14. — Loose stone polished and scratched under glacier-ice.
201. A glacier is not only busy grinding out a bed
for itself through the mountains ; it bears on its back
down the valley enormous quantities of fallen rock,
earth, and stones, which have tumbled from the cliffs
on either side. In this way blocks of rock as big as
a house may be carried for many miles, and dropped
where the ice melts. In the following figure (fig. 15) you
have a drawing of one of these huge masses of stone.
Thousands of tons of loose stones and mud are every
year moved on the ice from the far snowy moun-
84
SCIENCE PRIMERS. [circulation
tains a.way down into the valleys to which the glaciers
reach.
202. The largest glaciers in the world are those of the
polar regions. Norch Greenland, in truth, lies buried
under one great glacier, which pushes long tongues of
ice down the valleys and away out to sea. When a
glacier advances into the sea, portions of it break off
and float away as icebergs (fig. 16). So enormous are
Fig. is.-Erratic block, brought from the Alps by an ancient Glacier, and
dropped upon the Jura Mountains.
the glaciers in these cold tracts that the icebergs de-
rived from them often rise several hundred feet above
the waves which beat against their sides. And yet, in
all such cases, about seven times more of the ice is
immersed under water than the portion, large as it is,
which appears above. You can realize how this happens
if you take a piece of ice, put it in a tumbler of water,
and watch how much of it rises out of the water.
OF WATER.] PHYSICAL GEOGRAPHY.
85
Sunk deep in the sea, therefore, the icebergs float
to and fro until they melt, sometimes many hundreds
of miles away from the glaciers which supplied them.
203. You will come to learn afterwards that, once
upon a time, there were glaciers in Britain. You will
be able with your own eyes to see rocks which have
been ground down and scratched by the ice, and big
blocks of rock and piles of loose stones which the ice
Fig. 16. — Iceberg at Pea.
carried upon its surface. In Wales, and Cumberland,
in many parts of Scotland, and also in Ireland, these
and many other traces of the ice are to be found. So
that, in learning about glaciers, you are not merely
learning what takes place in other and distant lands,
you are gaining knowledge which you will be able
by and by to make good use of, even in your own
country.
86 SCIENCE PRIMERS. [the
THE SEA.
I. Grouping of Sea and Land.
204. Since we live on land, and are familiar with
the various shapes which the surface of the land
assumes, — plains, valleys, hills, mountains, and so on,
— we are apt to think that the land is the main part
of the globe. Many of us who live in the inland
parts of the country have never been off the land, nor
seen any larger sheet of water than a river or a lake,
or perhaps a large reservoir. And yet, if you were to
travel onward in any direction in Great Britain, you
would at last come to the edge of the land, and find a
vast expanse of water before you. If you took your
place in a ship, you could sail on that water com-
pletely round this country, and you would prove in
so doing that Britain is an island.
205. Suppose that instead of sailing round Britain,
which you could easily do in a few weeks, you were
to steer straight westward. You would have to travel
over the water for more than two thousand miles
before you reached any land again. Or, if you di-
rected your ship in a more southerly course, you might
sail on without seeing any land for months together,
until you came in sight of the ice-cliffs that border
the land round the South Pole. You would learn in
this way what an enormous extent of the surface of
the earth is occupied by water.
206. It has been ascertained that in reality the
water covers about three times more of the earth's
surface than the land does. ^Ve could not tell that
merely by what we can see from any part of this
country, or indeed of any country. It is because
SEA.] PHYSICAL GEOGRAPHY. 87
men have sailed round the world, and have crossed
it in many directions that the proportion of land and
water has come to be known.
207. Take a school-globe, and turn it slowly round
on its axis. You see at a glance how much larger the
surface of water is than the surface of land. But you
may notice several other interesting things about the
distribution of land and water.
208. In the first place, you will find that the water
is all connected together into one great mass, which
we call the sea. The land, on the other hand, is
much broken up by the way the sea runs into it ; and
some parts are cut off from the main mass of land,
so as to form islands in the sea. Britain is one of the
pieces of land so cut off.
209. In the second place, you cannot fail to notice
how much more land lies on the north than on the
south side of the equator. If you turn the globe so
that your eye shall look straight down on the site of
London, you will find that most of the land on the
globe comes into sight ; whereas, if you turn the
globe exactly round, and look straight down on the
area of New Zealand, you will see most of the sea.
London thus stands about the centre of the land-
hemisphere, midway among the countries of the earth.
And no doubt this central position has not been
without its influence in fostering the progress of
British commerce.
210. In the third place, you will notice that by the
way in which the masses of land are placed, parts of
the sea are to some extent separated from each other.
These masses of land are called continents, and
the wide sheets of water between are termed oceans.
88 SCIENCE PRIMERS. [the
Picture to yourselves that the surface of the solid
part of the earth is uneven, some portions rising into
broad swellings and ridges, others sinking into wide
hollows and basins. Now, into these hollows the sea
has been gathered, and only those upstanding parts
which rise above the level of the sea form the land.
211. In the foregoing parts of this little book men-
tion has often been made of the Sea. You have been
told that the moisture of the air comes in great part
from the sea ; that the rivers of the land are continually
flowing into the same reservoir of water, which is like-
wise the great basin into which all the soil which is
worn from the surface of the land is carried. We
must now look a little more closelv at some of the
more important features of the sea.
II. Whv the Sea is Salt.
2 12. When you come to examine the water of the
sea, you find that it differs from the water with which
you are famiHar on the land, inasmuch as it is salt.
It contains something which you do not notice in
ordinary spring or river water. If you take a drop of
clear spring-water, and allow it to evaporate from a
piece of glass, you will find no trace left behind. The
water of springs, as you have already leanit (Art. 117),
always contains some mineral substances dissolved in
it, and these not being capable of rising in vapour
are left behind when the water evaporates. But the
quantity of them in a single drop of water is so
minute that, when the drop dries up, it leaves no per-
ceptible speck or film. Take, however, a drop of sea-
water, and allow it to evaporate. You find a little
white point or film left behind, and on placing that
SEA.] PHYSICAL GEOGRAPHY. 89
film under a microscope you see it to consist of delicate
crystals of common or sea salt. It would not matter
from what ocean you took the drop of water, it would
still show the crystals of salt on being evaporated.
213. There are some other things besides common
salt in sea-water. But the salt is the most abundant,
and we need not trouble about the rest at present.
Now^, where did all this mineral matter in the sea
come from? The salt of the sea is all derived
from the waste of the rocks.
214. It has already been pointed out (Arts. 125,
132) how, both underground and on the surface of
the land, water is always dissolving out of the rocks
various mineral substances, of which salt is one.
Hence the water of springs and rivers contains salt,
and this is borne away into the sea. So that all
over the world there must be a vast quantity of salt
carried into the ocean every year.
215. The sea gives off again by evaporation as much
water as it receives from rain and from the rivers of
the land. But the salt carried into it remains behind.
If you take some salt water and evaporate it, the pure
water disappears, and the salt is left. So it is with
the sea. Streams are every day carrying fresh supplies
of salt into the sea. Every day, too, millions of tons
of water are passing from the ocean into vapour in
the atmosphere. The waters of the sea must con-
sequently be getting Salter by degrees. The process,
however, is an extremely slow one.
216. Although sea- water has probably been gradually
growins: in saltness ever since rivers first flowed into
the great sea, it is even now by no means as salt as
it might be. In the Atlantic Ocean, for example, the
go SCIENCE PRIMERS. [the
total quantity of the different salts amounts only to
about three and a half parts in every hundred parts
of water. But in the Dead Sea, which is extremely .
salt, the proportion is as much as twenty-four parts in
the hundred of water.
III. The Motions of the Sea.
217. Standing by the shore of any part of Britain,
and watching for a little the surface of the sea, you
noiice how restless it is. Even on the calmest summer
day, a slight ripple or a gentle heaving motion will be
seen ; at other times little wavelets curl towards the
land, and break in long lines upon the beach ; but
now and then, when storms arise, you may watch how
the water has been worked up into huge billows which,
crested with spray, come in, tossing and foaming, to
burst upon the shores.
218. Again, if you watch a little longer, you will find
that whether the sea is calm or rough, it does not
remain always at the same limit upon the beach. At
one part of the day the edge of the water reaches to
the upper part of the sloping beach ; some six hours
afterwards it has retired to the lower part. You may
watch it falling and rising, day after day, and year
after year, with so much regularity that its motion can
be predicted long beforehand. This ebb and flow of
the sea forms what are called tides.
219. If you cork up an empty bottle and throw it
into the sea, it will of course float. But it will not
remain long where it fell. It will begin to move
away, and may travel for a long distance until thrown
upon some shore again. Bottles cast upon mid-ocean
have been known to be carried in this way for many
SEA.] PHYSICAL GEOGRAPHY. 91
hundreds of miles. This surface -drift of the sea-
water corresponds generally 'with the direction in
which the prevalent winds blow.
220. But it is not merely the surface-water which
moves. You have learnt a little about icebergs
(Art. 202); and one fact about them which you must
remember is that, large as they may seem, there is
about seven times more of their mass below water than
above it. Now, it sometimes happens that an iceberg
is seen sailing on, even right in the face of a strong
wind. This shows that it is moving, not with the
wind, but with a strong under-current in the sea. In
short, the sea is found to be traversed by many
currents, some flowing from cold to warm regions,
and others from warm to cold.
221. Here, then, are four facts about the sea: —
I St, it has a restless surface, disturbed by ripples and
waves ; 2ndly, it is constantly heaving with the ebb
and flow of the tides ; 3rdly, its surface-waters drift
with the wind ; and 4thly, it possesses currents like
the atmosphere.
222. For the present it will be enough if we learn
something regarding the first of these facts — the
waves of the sea.
223. Here again you may profitably illustrate by
familiar objects what goes on upon so vast a scale in
nature. Take a basin, or a long trough of water, and
blow upon the water at one edge. You throw its
surface into ripples, which, as you will observe, start
from the place where your breath first hits the water
and roll onward until they break in little wavelets
upon the opposite margin of the basin.
224. What you do in a small way is the same action
9
92 SCIENCE PRIMERS. [the
by which the waves of j;he sea are formed. All these
disturbances of the smoothness of the sea are due to
disturbances of the air. Wind acts upon the water of
the sea as your breath does on that of the basin.
Striking the surface, it throws the water into ripples
or undulations, and in continuing to blow along the
surface it gives these additional force, until driven
on by a furious gale they grow into huge billows.
225. When waves roll in on the land, they break
one after another upon the shore, as your ripples
break upon the side of the basin. And they continue
to roll in after the wind has fallen, in the same v^^ay
that the ripples in the basin will go on curling for a
little after you have ceased to blow. The surface of
the sea, like that of water generally, is very sensitive.
If it is thrown into undulations, it does not become
motionless the moment the cause of disturbance has
passed away, but continues moving in the same way,
but in a gradually lessening degree, until it comes
to rest.
226. The restlessness of the surface of the sea
becomes in this way a reflection of the restlessness of
the air. It is the constant moving to and fro of cur-
rents of air, either gentle or violent, which roughens
the sea with waves. When the air for a time is calm
above, the sea sleeps peacefully below \ when the sky
darkens, and a tempest bursts forth, the sea is lashed
into waves, which roll in and break with enormous
force upon the land.
227. You have heard, perhaps you have even seen,
something of the destruction which is worked by the
waves of the sea. Every year piers and sea-walls are
broken down, pieces of the coast are washed away.
SEA.]
PHYSICAL GEOGRAPHY.
93
and the shores are strewn with the wret.k of ships.
So that, besides all the waste which the surface of the
land undergoes from rain, and frost, and streams,
there is another form of destruction going on along
the coast-line.
228. On rocky shores the different stages in the
eating away of the land by the sea can sometimes
be strikingly seen. Above the beach perhaps rises a
.cliff, sorely battered about its base by the ceaseless
.-\
Fig. 17. — Coast-line worn by the S«a.
grinding of the waves. Here and there a cavern has
been drilled in the solid wall, or a tunnel has been
driven through some ])rojecting headland. Not f^ir off
we may note a tall buttress of rock, once a part of the
main cliff, but now separated from it by the falhng in
and removal of the connecting archway. And then,
further off from the cliff, isolated, half-tide rocks
rise to show wliere still older detached buttresses
94 SCIENCE PRIMERS. [the
Stood ; while away out in the sea the dash of breakers
marks the site of some sunken reef, in which we see
the reUcs of a still more ancient coast-line. On su6h
a shore the whole process whereby the sea eats into
the land seems to be laid open to our eyes.
229. On some parts of the coast-line of the east of
England, where the rock is easily worn away, the
sea advances on the land at a rate of two or three
feet every year. Towns and villages which existed a
i^"^ centuries ago have one by one disappeared, and
their sites are now a long way out under the restless
waters of the North Sea. On the west coast of Ire-
land and Scotland, however, where the rocks are
usually hard and resisting, the rate of waste has been
comparatively small.
230. It would be worth your while the first time
you happen to be at the coast to ascertain what
means the sea takes to waste the land. This you can
easily do by watching what happens on a rocky beach.
Get to some sandy or gravelly part of the beach,
over which the waves are breaking, and keep your
eye on the water when it runs back after a wave
has burst. You see all the grains of gravel and sand
hurrying down the slope with the water; and if the
gravel happens to be coarse, it makes a harsh grating
noise as its stones rub agairst each other — a noise
sometimes loud enough to be heard miles away. As
the next wave comes curling along, you will mark
that the sand and gravel, after slackening their
downward pace, are caught up by the bottom of the
advancing wave and dragged up the beach again,
only to be hurried down once more as the water
retires to allow another wave to do the same work.
SE.\.] PHYSICAL GEOGRAPHY. 95
231. By this continual up and do\Mi movement of
the water, the sand and stones on the beach are
kept grinding against each other, as in a mill.
Consequently they are worn away. The stones
become smaller, until they pass into mere sand,
and the sand, growing finer, is swept away out
to sea and laid down at the bottom.
232. But not only the loose materials on the
shore suffer in this way an incessant wear and tear,
the solid rocks underneath, wherever they come to
the surface, are ground down in the same process.
When the Avaves dash against a cliff they hurl the
loose stones forward, and batter the rocks ^^'ith them.
Here and there in some softer part, as in some crevice
of the cliff, these stones gather together, and when the
sea runs high they are kept whirling and grinding at
the base of the cliff till, in the end, a cave is actually
bored by the sea in the solid rock, very much in
the same way as, you remember (Art. 174), we saw
that holes are bored by a river in the bed of its
channel. The stones of course are ground to sand
in the process, but their place is supplied by others
swept up by the waves. If you enter one of these
sea-caves when the water is low, you will see how
smoothed and polished its sides and roof are, and
how well rounded and worn are the stones lying
on its floor.
IV. The Bottom of the Sea.
233. So far as we know, the bottom of the sea is
ver)' much like the surface of the land. It has heights
and hollows, lines of valleys and ranges of hills. We
cannot see down to the bottom where the water is
96 SCIENCE PRIMERS. [the
very deep, but we can let down a long line with a
weight tied to the end of it, and find out both how
deep the water is, and what is the nature of the
bottom, whether rock or gravel, sand, mud, or shells.
This measuring of the depths of the water is called
Sounding, and the weight at the end oi the line
goes by the name of the Sounding-lead.
234. Soundings have been made over many parts
of the sea, and something is now known about its
bottom, though much still remains to be discovered.
The Atlantic Ocean is the best known. In sounding
it, before laying down the telegraphic cable which
stretches across under the sea from this country to
America, a depth of 14,500 feet, or two miles and
three-quarters, was reached. But between the Azores
and the Bermudas a sounding has been obtained of
seven miles and a half. If you could lift up the
Himalaya mountains, which are the highest on the
globe, reaching a height of 29,000 feet above the sea,
and set them down in the deepest part of the Atlantic,
they would not only sink out of sight, but their tops
would actually be about two miles below the surface.
235. A great part of the wide sea must be one or
two miles deep. But it is not all so deep as that, for
even in mid-ocean some parts of its bottom rise up to
the surface and form islands. As a rule it deepens in
the tracts furthest from land, and shallows towards
the land. Hence those parts of the sea which run
in among islands and promontories are, for the most
part, comparatively shallow. To the west of the
island of Great Britain, stretches the wide Atlantic
Ocean ; to the east lies the much smaller North Sea ;
the former soon getting very deep as we sail west-
SEA.] PHYSICAL GEOGRAPHY. 97
wards across it, the latter never deepening much even
over its middle parts, which are nowhere so much
as 400 feet below the surface. You may get some
notion of the shallowness of the sea between this
country and France, when you are told that if you
could lift St. Paul's cathedral from London, and set
it down in the middle of the Strait of Dover, more
than a half of the building would be out of the water.
236. You may readily enough understand how it is
that soundings are made, though you can see how
difficult it must be to work- a sounding-line several
miles long. Yet men are able not only to measure
the depth of the water, but by means of the instru-
ment called a dredge, to bring up bucketfuls of
whatever may be lying on the sea-floor, from even the
deepest parts of the ocean. In this way during the
last few years a great deal of additional knowledge
has been gathered as to the nature of the sea-floor,
and the kind of plants and animals which live there.
We now know that even in some of the deepest
places which have yet been dredged there is plenty
of animal life, such as shells, corals, star-fishes, and
still more humble creatures.
237. In earlier parts of this book we have traced
some of the changes which from day to day take
place upon the surface of the land. Let us now try
to watch some of those which go on upon the floor of
the sea. We cannot, indeed, examine the sea-bottom
with anything like the same minuteness as the surface
of the land. Yet a great deal may be learnt regard-
ing it.
22,'^. If you put together some of the acts with
which we have been dealing in the foregoing
98 SCIENCE PRIMERS. [the
Lessons, you may for yourselves make out some of
the most important changes which are in progress
on the floor of the sea. For example, try to think
what must become of all the wasted rock which is
every year removed from the surface of the land.
It is carried into the sea by streams, as you have
now learnt. But what happens to it when it gets
there? From the time when it was loosened from
the sides of the mountains, hills, or valleys, this
decomposed material has been seeking, like water,
to reach a lower level. On reaching the hollows of
the sea -bottom it cannot descend any further, but
must necessarily accumulate there.
239. It is evident, then, that between the floor of
the sea and the surface of the land, there must be
this great difl"erence : that. whereas the land is under-
going a continual destruction of its surface, from
mountain-crest to sea-shore, the sea-bottom, on the
other hand, is constantly receiving fresh materials
on its surface. The one is increased in proportion
as the other is diminished. So that even without
knowing anything regarding what men have found
out by means of deep soundings, you could confi-
dently assert that every year there must be vast
quantities of gravel, sand, and mud laid down
upon the floor of the sea, because you know that
these materials are worn away from the land.
240. Again, you have learnt that the restless agita-
tion of the sea is due to movements of the air, and
that the destruction which the sea can effect on the
land is due chiefly to the action of the waves caused
by wind. But this action must be merely a surface
one. The influence of the waves cannot reach to
SEA.] PHYSICAL GEOGRAPHY. 99
the bottom of the deep sea. Consequently that
bottom lies beyond the reach of the various kinds
of destruction which so alter the face of the land.
The materials which are derived from the waste of
the land can lie on the sea-floor without further dis-
turbance than they may suffer from the quiet flow of
such ocean currents as touch the bottom.
241. In what way, then, are the gravel, sand, and
mud disposed of when they reach the sea ?
242. As these materials are all brought from the
land, they accumulate on those parts of the sea-floor
which border the land, rather than at a distance. We
may expect to find banks of sand and gravel in
shallow seas and near land, but not in the middle of
the ocean.
243. You may form some notion, on a small scale,
as to how the materials are arranged on the sea-
bottom, by examining the channel of a river in a
season of drought. At one place, where the current
has been strong, there may be a bank of gravel ; at
another place, where the currents of the river have
met, you will find, perhaps, a ridge of sand which
they have heaped up ; while in those places where
the flow of the stream has been more gentle, the
channel may be covered with a layer of fine silt or
mud. You remember that a muddy river may be
made to deposit its mud if it overflows its banks so
far as to spread over flat land which checks its flow
(Art. 178).
244. The more powerful a current of water, the
larger will be the stones it can move along. Hence
coarse gravel is not likely to be found over the bottom
of the sea, except near the land, where the waves can
loo SCIENCE PRIMERS. [the
sweep it out into the path of strong sea-currents.
Sand will be carried further out, and laid down in great
sheets, or in banks. The finer mud and silt may be
borne by currents for hundreds of miles before at
last settling down upon the sea-bottom.
245. In this way, according to the nearness of the
land and the strength of the ocean-currents, the sand,
mud, and gravel worn from the land are spread out
in vast sheets and banks over the bottom of the sea.
246. But the sea is full of life, both of plants
and animals. These organisms die, and their re-
mains necessarily get mixed up with the different
materials laid down upon the sea-floor. So that,
besides the mere sand and mud, great numbers of
shells, corals, and the harder parts of other sea-
creatures must be buried there, as generation after
generation comes and goes.
247. It often happens that on parts of the sea-bed
the remains of some of these animals are so abun-
dant that they themselves form thick and wide-
spread deposits. Oysters, for example, grow thickly
together ; and their shells, mingled with those of
other similar creatures, form what are called shell -
banks. In the Pacific and the Indian Oceans a
little animal, called the coral-polyp, secretes a hard
limy skeleton from the sea-water ; and as millions of
these polyps grow together, they form great reefs of
solid rock, which are sometimes, as in the Great
Barrier Reef of Australia, hundreds of feet thick
and a thousand miles long. It is by means of the
growth of these animals that those wonderful rings
of coral-rock or Coral-islands (Fig. 18) are formed
in the middle of the ocean. Again, a great part of
SEA.]
PHYSICAL GEOGRAPHY.
lOI
the bed of the Atlantic Ocean is covered with fine
mud, which on examination is found to consist almost
wholly of the remains of very minute animals called
Foraminifera.
Fig. i8. — Island formed by the Growth of Coral.
248. Over the bottom of the sea, therefore, great
beds of sand and mud, mingled with the remains
of plants and animals, are always accumulating. If
now this bottom could be raised up above the sea-
level, even though the sand and mud should get as
dry and hard as any rock among the hills, you would
be able to say with certainty that they had once been
under the sea, because you would find in them tlie
shells and other remains of marine animals.
249. You will afterwards learn when you come to
the science of Geology that this raising of the sea-
bottom has often taken place in ancient times. You
will find most of the rocks of our hills and valleys to
have been originally laid down in the sea, where they
were formed out of sand and mud dropped on the sea-
floor, just as sand and mud are carried out to sea and
laid down there now. And in these rocks, not merely
near the shore, but far inland, in quarries or ravines.
I02 SCIENCE PRIMERS. [inside of
or the sides and even the tops of hills, you will be
able to pick out the skeletons and fragments of
the various sea-creatures which were living in the
old seas.'
250, Since the bottom of the sea forms the great
receptacle into which the mouldered remains of the
surface of the land are continually carried, it is plain
that if this state of things were to go on without
modification or hindrance, in the end the whole of
the solid land would be worn away, and its remains
would be spread out on the sea-floor, leaving one
vast ocean to roll round the globe.
251. But there is in nature another force which
here comes into play to retard the destruction of the
land. We must in the remaining Lessons of this
book consider what this force is, and how it works.
THE INSIDE OF THE EARTH.
252. In the foregoing pages your attention has
been given to the surface of the earth, and what
goes on there. Let us now consider for a little
w^hat can be learnt regarding the inside of the earth.
253. It may seem, at first, as if it were hopeless
that man should ever know anything about the earth's
interior. Just think what a huge ball this globe of
ours is, and you will see that after all, in living and
moving over its surface, we are merely like flies walk-
ing over a great hill. All that can be seen from the
top of the highest mountain to the bottom of the
THE EARTH.] PHYSICAL GEOGRAPHY. 103
deepest mine is not more in comparison than the
mere varnish on the outside of a school-globe. And
yet a good deal can be learnt as to what takes place
within the earth. Here and there, in different coun-
tries, there are places where communication exists
between the interior and the surface ; and it is from
such places that much of our information on this
subject is derived.
254. You have, no doubt, read of Volcanoes or
Burning-mountains (fig. 19). These are among
the most important of the channels of communication
with the interior.
255. Let us suppose that you were to visit one of
these volcanoes just before what is called ''an eruption."
As you approach it, you see a conical mountain, seem-
ingly with its top cut off. From this truncated sum-
mit a white cloud rises. But it is not quite such a
cloud as you would see on a hill-top in this countr)\
For as you watch it you notice that it rises out of
the top of the mountain, even though there are no
clouds to be seen anywhere else. Ascending from
the vegetation of the lower grounds, you find the
slopes to consist partly of loose stones and ashes,
partly of rough black sheets of rock, like the slags of
an iron furnace. As you get nearer the top the ground
feels hot, and puffs of steam, together with stifling
vapours, come out of it here and there. At last you
reach the summit, and there what seemed a level top
is seen to be in reality a great basin, with steep
walls descending into the depths of the mountain.
Screening your face as well as possible from the hot
gases which almost choke you, you creep to the top
of this basin, and look down into it. Far below, at
10
i04
SCIENCE PRIMERS.
[inside of
the base of the rough red and yellow cliffs which
form its sides, lies a pool of some liquid, glowing with
a white heat, though covered for the most part with
a black crust like that seen on the outside of the
mountain during the ascent. From this fiery pool jets
of the red-hot liquid are jerked out every now and
Fig. 19.— View of a ^^olcano. Mount Vesuvius as it -ppears at the
present time, when viewed from the south.
then, stones and dust are cast up into the air, and fall
back again, and clouds of steam ascend from the same
source and form the uprising cloud which is seen from
a great distance hanging over the mountain.
' 256. This caldron-shaped hollow on the summit of
the mountain is the Crater. The intensely heated
liquid in the sputtering boiling pool at its bottom is
melted rock or Lava. And the fragmentary materials
■ — ashes, dust, cinders, and stones — thrown out, are
torn from the hardened sides and bottom of the crater
THE EARTH.] PHYSICAL GEOGRAPHY. 105
by the violence of the explosions with which the
gases and steam escape.
257. The hot air and steam, and the melted mass
at the bottom of the crater, show that there must be
some source of intense heat underneath. And as the
heat has been coming out for hundreds, or even thou-
sands of years, it must exist there in great abundance.
258. But it is when the volcano appears in active
eruption that the power of this underground heat
shows itself most markedly. For a day or two before-
hand, the ground around the mountain trembles. At
length, in a series of violent explosions, the heart of
the volcano is torn open, and perhaps its upper part
is blown into the air. Huge clouds of steam roll
away up into the air, mingled with fine dust and red-
hot stones. The heavier stones fall back again into
the crater or on the outer slopes of the mountain, but
the finer ashes come out in such quantity, as sometimes
to darken the sky for many miles round, and to settle
down over the surrounding country as a thick cover-
ing. Streams of white-hot molten lava run down the
outside of the mountain, and descend even to the
gardens and houses at the base, burning up or over-
flowing whatever lies in their path. This state of
matters continues for days or weeks, until the volcano
exhausts itself, and then a time of comparative quiet
comes when only steam, hot vapours, and gases are
given off.
259. About 1800 years ago, there was a mountain
near Naples shaped like a volcano, and with a large
crater covered with brushwood (fig. 20). No one had
ever seen anv steam, or ashes, or lava come from it,
and the people did not imagine it to be a volcano, like
io6
SCIENCE PRIMERS.
[inside of
some other mountains in that part of Europe. They
had built villages and towns around its base, and
their district, from its beauty and soft climate, used
to attract wealthy Romans to build villas there.
But at last, after hardly any warning, the whole of
the higher part of the m.ountain was blown into
the air with terrific explosions. Such showers of
fine ashes fell for miles around, that the sky was as
dark as midnight. Day and night the ashes and
Fig. 20. -Vesuvius as it appeared before Pompeii was destroyed.
Stones descended on the surrounding country ; many
of the inhabitants were killed, either by stones falling
on them, or from suffocation by the dust. When at
last the eruption ceased, the district, which had
before drawn visitors from all parts of the old world,
was found to be a mere desert of grey dust and
stones. Towns and villages, vineyards and gardens,
were all buried. Of the towns, the two most noted
were called Herculaneum and Pompeii. So com-
THE EARTH.] PHYSICAL GEOGRAPHY. 107
pletely did they disappear, that, although important
places at the time, their very sites were forgotten,
and only by accident, after the lapse of some fifteen
hundred years, were they discovered. Excavations
have since that time been carried on, the hardened
volcanic accumulations have been removed from the
old city, and you can now walk through the streets
of Pompeii again, with their roofless dwelling-houses
and shops, theatres and temples, and mark on the
causeway the deep ruts worn by the carriage wheels
of the Pompeians eighteen centuries ago. Beyond
the walls of the now silent city rises Mount Vesuvius,
with its smoking crater, covering one-half of the old
mountain which was blown up when Pompeii dis-
appeared (See fig. 19.)
260. Volcanoes, then, mark the position of some of
the holes or orifices, whereby heated materials from
the inside of the earth are thrown up to the surface.
They occur in all quarters of the globe. In Europe,
besides Mount Vesuvius, which has been more or less
active since it was formed, Etna, Stromboli, and
other smaller volcanoes, occur in the basin of the
Mediterranean, while far to the north-west some active
volcanoes rise amid the snows and glaciers of Iceland.
In America a chain of huge volcanoes stretches down
the range of mountains which rises from the western
margin of the continent. In Asia they are thickly
grouped together in Java and some of the surround-
ing islands ; and stretch thence through Japan and
the Aleutian Isles, to the extremitv of North America.
If you trace this distribution upon the map, you will
see that the Pacific Ocean is girded all round with
volcanoes.
io8 SCIENCE PRIMERS. [inside of
261. Since these openings into the interior of the
earth are so numerous over the surface, we may
conclude that this interior is intensely hot. But we
have other proofs of this internal heat. In many
countries hot springs rise to the surface. Even in
England, which is a long way from any active
volcano, the water of the wells of Eath is quite
warm (120° Fahr.). It is known, too, that in all coun-
tries the heat increases as we descend into the earth.
The deeper a mine the warmer are the rocks and
air at its bottom. If the heat continues to increase
in the same proportion, the locks must be red hot
at no great distance beneath us,
262. It is not merely by volcanoes and hot-springs,
however, that the internal heat of the earth affects
the surface. The solid ground is made to tremble,
or is rent asunder, or upheaved or let down. You
have probably heard or read of earthquakes :
those shakings of the ground, which, when they are at
their worst, crack the ground open, throw down trees
and buildings, and bury hundreds or thousands of
people in the ruins. Earthquakes are most common
in or near those countries where active volcanoes exist.
They frequently take place just before a volcanic
eruption.
263. Some parts of the land are slowly rising out
of the sea ; rocks, which used always to be covered .
by the tides, come to be wholly beyond their limits ;
while others, which used never to be seen at all,
begin one by one to show their heads above water.
On the other hand some tracts are slowly sinking ;
piers, sea-walls, and other old landmarks on the
beach, are one after apother enveloped by the sea
THE EARTH.] PHYSICAL GEOGRAPHY. 109
as it encroaches further and higher on the land.
These movements, whether in an upward or down-
ward direction, are likewise due in some way to the
internal heat.
264. Now when you reflect upon these various
changes you will see that through the agency of
this same internal heat land is preserved upon the
face of the earth. If rain and frost, rivers, glaciers,
and the sea were to go on wearing down the
surface of the land continually without any counter-
balancing kind of action, the land would neces-
sarily in the end disappear, and indeed would have
disappeared long ago. But owing to the pushing out
of some parts of the earth's surface by the move-
ments of the heated materials inside, portions of the
land are raised to a higher level, while parts of tlie
bed of the sea are actually upheaved so as to form
land.
265. Tliis kind of elevation has happened many
times in all quarters of the globe. As already men-
tioned (Art. 249), most of our hills and valleys are
formed of rocks, which were originally laid down on
the bottom of the sea, and have been subsequently
raised into land.
CONCLUSION.
266. In conclusion, let us sum up the leading
features of the foregoing Lessons.
267. This earth of ours is the scene of continual
movement and change. The atmos])here which
encircles it is continually in motion, diffusing heat,
light, and vapour. From the sea and from the waters
no SCIENCE PRIMERS. [conclusion.
of the land, vapour is constantly passing into the air,
whence, condensed into clouds, rain and snow, it
descends again to the earth. All over the surface of
the land the water which falls from the sky courses
seawards in brooks and rivers, bearing into the great
deep the materials which are worn away from the land.
Water is thus ceaselessly circulating between the air,
the land, and the sea. The sea, too, is never at
rest. Its waves gnaw the edges of the land, and its
currents sweep round the globe. Into its depths the
spoils of the land are borne, there to gather into rocks,
out of which new islands and continents will even-
tually be formed. Lastly, inside the earth is lodged a
vast store of heat by which the surface is shaken,
rent open, upraised or depressed. Thus while old
land is submerged beneath the sea, new tracts are up-
heaved, to be clothed with vegetation and peopled
with animals, and to form a fitting . abode for man
himself.
268. This world is not a living being, like a plant
or an animal, and yet you must now see that there
is a sense in which we may speak of it as such. The
circulation of air and water, the interchange of sea
and land \ in short, the system of endless and con-
tinual movement by which the face of the globe is
day by day altered and renewed, may well be called
the Life of the Earth.
QUESTIONS.
THE SHAPE OF THE EARTH, p. 8.
1. What is the first impression we have of the shape of the
Earth ?
2. How could you show in the interior of a level country that
the apparent plain is really part of the surface of a globe?
3. Prove the same conclusion from what may be seen on the
sea-coast.
4. How has the shape of the earth been tested by "circum-
navigators ? ' '
5. Show how the gentle curvature indicates the size of the
globe.
6. How long would a railway train moving at a rate of thirty
miles an hour take to go round the earth ?
DAY AND NIGHT, p. 13.
1. Whence does the earth derive its surface-heat and light ?
2. What was the ancient belief as to the relative positions of
the earth, sun, moon, and stars?
3. Are there any traces of this early belief ctill to be found in
our everyday speech ?
4. What is the real relation of tbe sun to the earth ?
5. The succession of day and night appears as if it were due
•to the movement of the sun across the sky ; illustrate how it is
really caused by the motion of the earth.
6. What is meant by the terms axis of rotation, north pole
and south pole ?
7. In what direction is the earth rotating ? How is this indi-
cated by day and at night respectively?
8. What is the earth's motion of reziolution ?
9. In what time does the earth perform a complete revolu-
tion ?
10. Show how the movements of the earth determine our
divisions of time.
112 SCIENCE PRIMERS. [questions.
THE AIR.
I. What the Air is made of, p. i6.
1. What is meant by the term Atmosphere ?
2. Of what materials is the air mainly composed ?
3. Besides the two chief gases, name some other substances
alA^ays present in the air,
4. How may the presence of visible particles be shown ?
5. What is water-vapour? [See art. 73.] Show by any
familiar example how it may be invisibly dissolved in the air.
[See art, 71.]
6. In what proportion does carbonic acid gas occur in the
air?
7. Show how important this material is in relation to the
growth of plants and animals.
II. The Warming and Cooling of the Air, p, ig.
1. In what ways are we made sensible of the presence of the
air?
2. Why do we feel cold when we pass from a warm room into
the outer air in winter?
3. The sun is always radiating heat to the earth ; why then
should there be alternations of heat and cold in the air ?
4. Does the atmosphere allow the whole of the sun's heat-rays
to pass through it to the surface of the earth ?
5. Why is the sun's heat less felt in the morning and in the
evening than at noon ?
6. Why is night so much colder than day ?
7. Why is summer warmer than winter ?
8. Why is it that cloudy days are not always or necessarily
cold ?
9. Since the air absorbs only part of the heat of the sun which
passes through it to the earth's surface, how is it chiefly warmed
and how cooled? [See art. 64.]
ID. What prevents excessive loss of heat at night by radia-
tion ?
11. Why are the nights often felt to be so cold in warm
countries ?
12. Why are cloudy nights usually warmer than clear ones?
III. What happens when Air is warmed or cooled.
Wind, p. 24.
I. Whether is warm or cold air the heavier, and why?
QUESTIONS.] PHYSICAL GEOGRAPHY. 113
2. What is the general effect of difference of density in causing
movement of the air ?
3. How do a red-hot poker and a common fire-place illus-
trate this movement ?
4. How does wind arise from the unequal heating of the
earth's surface ?
5. Explain the nature and origin of land and sea breezes.
6. Which is the hottest belt of the earth's surface, and why ?
7. Explain the nature and origin of the trade winds.
8. How does water-vapour cause movements in the atmo-
sphere ?
*
IV. The Vapour in the Air. Evaporation and Con-
densation, p. 27.
1. Explain why a film of mist appears on a cold glass when
brought into a warm room,
2. How does the capacity of the air to retain water-vapour
vary according to temperature ^
3. Why does a film of mist appear upon a mirror or other
cold surface when it is breathed on, and what is the explanation
of the cloud which issues from one's mouth with every breath in
cold weather.
4. What is the dew-point ?
5. How is the vapour of water brought into the air?
6. At what times is evaporation most and least vigorous ?
7. Explain the cause of the chill that is felt when a drop of
water is evaporated on the back of the hand ?
V. Dew, Mist, Clouds, p. 31.
1. Give some exam.ples of the condensation of vapour.
2. Explain the formation of dew.
3. Show how mists are formed upon mountains
4. Explain the origin of the fog often seen rising after sunset
from the surface of a river.
5. Explain the formation of clouds.
VI. Where Rain and Snow come from, p. 35.
1. In what ways do clouds disappear from the sky ?
2. "Explain the formation and fall of rain.
3. Under what different forms does water present itself?
4. What is ice and when is it formed ?
5. What is snow ? Describe a snow-flake.
114 SCIENCE PRIMERS. [questions.
6. What are hail and sleet ?
7. Describe the circulation of water between the air and the
earth.
THE CIRCULATION OF WATER ON THE LAND.
I. What becomes of the Rain, p. 39
1. Why do not seas, lakes, and rivers, become visibly less,
seeing that they lose so much water by evaporation ?
2. What part does the sea play in supplying the air with
moisture ?
3. What becomes of that part of the rain which falls into the
sea?
4. How much rain is estimated to fall annually upon the
British Isles?
5. How is the rain which falls upon land disposed of? [See
art. 106.]
6. How may it be shown that a considerable quantity of rain
sinks into the ground, and yet that this quantity is not perma-
nently removed from the circulation ?
II. How Springs are formed, p. 42.
1. How do sand and clay differ from each other in regard to
the passage of water through them?
2. How does this difference affect the kinds of soil?
3. What inference as to the movements of the underground
water, may be drawn from the fact that water gathers in any
deep hole or quarry which may be dug out of the ground?
4. What natural channels are provided for the passage of
water, even through very hard rocks ?
5. Explain the occurrence of boggy places in hilly ground.
6. What are springs?
7. Explain why springs issue from between beds of rock along
the sides of valleys.
8. Explain the origin of deep-seated springs.
9. How is the underground circulation of water shown by
wells, mines, and pits ?
III. The work of Water Underground, p. 47.
1. Does clear spring- water contain anything else than water ?
How may this be answered practically ?
2. What common solutions show that clear transparent water
may contain a good deal of foreign matter invisible to the eye ?
QUESTIONS.] PHYSICAL GEOGRAPHY. iij
3. Whence must the substances dissolved in spring-water be
derived ?
4. What part does rain play in regard to the purification of
the air ?
5. Whence does rain-water derive the carbonic acid which it
carries below the soil ?
6. What effect has water containing carbonic acid on many
rocks ?
7. Explain this action of water in limestone countries.
8. What is the difference between hard and soft water?
9. Are the substances carried up from below in spring- water
of any service in the growth of plants and animals ?
10. What is the origin of underground tunnels and caverns ?
IV. How the Surface of the Earth crumbles away,
p. 51.
1. What change usually takes place upon masonry after it has
been exposed for a time to the air?
2. Show how a similar change can be obsers'ed elsewhere than
in human erections.
3. Explain the part taken by carbonic acid in the crumbling
of the rocks at the surface of the earth.
4. Explain the eftect of the oxygen in rain-water upon iron
and on many rocks.
5. Explain the action of frost in promoting the crumbling of
soil and the splitting up of rocks.
6. What is the effect of rapid extremes of heat and cold upon
rocks ?
7. State the general result of all these destructive agents
upon the surface of the land, and show how their action is bene-
ficial in making the earth a fit dwelling-place for plants and
animals.
V. What becomes of the Crumbled Parts of Rocks.
How Soil is made, p. 58.
1. What is common garden soil made of?
2. What is meant by the chemical action of rain ?
3. Explain the mechanical action of rain.
4. What is the nature of the process by which soil is made?
5. Explain how soil is continually renewed.
6. Show how plants lend their help in the making of soil.
7. What part do common earth-worms play in the process?
8. In what sense may it be said that the general surface of
the land is continually moving towards the sea ?
9. How do brooks and rivers illustrate the extent to which the
surface of the land is mouldering ?
11
ii6 SCIENCE PRIMERS. [questions.
VI. Brooks and Rivers. Their Origin, p. 62.
1. Describe the formation of miniature brooks and rivers on
a sloping roadway during a heavy shower of rain.
2. Why do streams flow?
3. What are lakes ?
4. Why does the rain run off the surface of the land in
runnels, brooks, and rivers ?
5. How are the innumerable brooks of the high ground
disposed of as they descend towards the lo ^ver country ?
6. What is meant by a water-shed ?
7. Why do rivers continue to flow even in dry weather ?
8. Why are some rivers, such as the Rhine, most swollen in
summer?
9. What becomes of all the surplus drainage of the land ?
VII. Brooks and Rivers. Their Work, p. 68.
1. Give an illustration of the vast amount of invisible material
carried, in chemical solution, by a river to the sea.
2. Why are rivers discoloured during floods ?
3. What is the origin of the gravel and blocks of stone in
the bed of a stream, and why are the stones usually rounded ?
4. What are pot-holes ?
5. How have river gorges and ravines been formed ?
6. Describe the bed of a river when the water is low.
7. Explain the origin of the flat terraces bordering a river.
8. Describe a delta, and show how it may be formed at the
mouth of a river, in a lake, or in the sea.
9. What becomes of the mud and sand which are carried past
the delta?
VIII. Snowfields and Glaciers, p. 75.
1. What is meant by the snow-line ?
2. What is its height at the equator and in the polar regions ?
3. Why does snow remain perpetual above the snow-line ?
4. In what way does the snow below the snow-line disappear ?
5. How may the sudden melting of snow prove very destruc-
tive?
6. What becomes of the mass of snow which accumulates
above the snow-line ?
7. Describe the formation of a glacier.
8. What becomes of a glacier as it descends its valley ?
9. What are moraines ?
10. How do stones and earth get under the ice of a glacier?
11. What use does the glacier make of these stones and
particles of earth and sand ?
Q-JESiiuNS.] PHYSICAL GEOGRAPHY. 117
12. Why is the river of water muddy which escapes from the
end of a glacier ?
13. Where do the largest glaciers exist ?
14. Explain the formation of icebergs.
15. What proofs have been found that glaciers once existed
in countries such as Britain, where they no longer occur?
THE SEA.
I. Grouping of Sea and Land, p. 86.
1. What are the proportions of land and water on the earths
surface ?
2. Mention the broad difference between sea and land in the
way they are distributed over the globe.
3. On which side of the equator does most of the land lie ?
4. What part of the earth's surface lies in the centre of the
land hemisphere ?
5. What are continents and islands?
6. What are oceans ?
II. Why the Sea is Salt, p. 88.
1. In what familiar respect does the water of the sea differ
from that of ordinary springs and rivers ?
2. What happens when a drop of sea-water is evaporated on
a piece of glass ?
3. Whence h -s the miner<?l matter in sea- water come ?
4. What is the relative salness of the Atlantic Ocean and t^e
Dead Sea?
III. The Motions of the Sea, p. 90.
1. What is the commonest and most obvious form of motion
in the sea ?
2. How does the ebb and flow of the tides show itself on a
sloping beach ?
3. What is surface drift, and how is it often indicated ?
4. What are currents in the sea, and how are they sometimes
made evident ?
5. How may a basin or trough of water be made to illustrate
the formation of waves ?
6. What is the connection between movements of the air and
ripples or waves on the sea ?
7. What general effect have waves on the edge of the land
exposed to their influence?
ii8 SCIENCE PRIMERS. [QUEbTicss.
8. Explain the process by which gravel and sand are ground
down by the waves upon the beach.
9. How do the waves wear down a rocky coast ?
IV. The Bottom of the Sea, p. 95.
1. What is the general character of the sea-floor as compared
with the surface of the land ?
2. How is our information regarding the bottom of the deep
sea obtained ?
3. What was found to be the depth of the Atlantic Ocean
Avhen soundings were made for the telegraphic cable between
Britain and America ?
4. What is the greatest depth that has yet been observed in
the Atlantic, and where does it occur ?
5. What is the depth of a great part of the sea?
6. Which are usually the deepest and which the shallowest
parts of the sea ?
7 What is the depth of the deeper parts of the North Sea?
8. How much of St. PauFs Cathedral in London would be
submerged were it placed in the middle of the -Straits of
Dover ?
9. What is a di-edge, and what use is made of it ?
10. What light has been obtained by means of the dredge
regarding the living things of the deep sea bottom ?
n. Mention an important difference between the crumbling
land-surface described in a former lesson [arts. 123 — 142], and
the bottom of the sea.
12. To what part of the sea is the destructive action of the
waves limited ?
13. How are the mud, earth, sand, and gravel disposed of
M^hich the sea obtains from the crumblinfj surface of the land?
14. What becomes of the remains of the shells, corals, and
other creatures on the sea-floor ?
15. What are shell-banks ? ,
16. What are coral-reefs and coral-islands, and how are they
formed ?
17. What is the nature of the mud which covers a great part
of the bed of the Atlantic?
18. How could you be certain that some rocks must once have
been under the sea ?
THE INSIDE OF THE EARTH, p. 102
I. Does the distance from the top of the highest mountain
to the bottom of the deepest mine bear a large proportion to
the diameter of the whole globe ?
QUESTIONS.] PHYSICAL GEOGRAPHY. 119
2. What is a volcano ?
3. What various materials are 'throNvn out by a volcano ?
4. What evidence do these materials furnish as to the condition
of the earth's interior?
5. Describe a volcanic eruption.
6. What has been the history of Vesuvius?
7. State the position of some of the volcanoes of Europe,
America, and Asia.
8. What evidence do hot springs bring to bear upon the
state of the iniernal parts of our globe ?
9. What has been observed regarding temperature as we
descend into the earth, and what inference has been drawn
from it ?
10. What are earthquakes? Where are they most frequent ?
11. Mention any facts which show that different pans of the
earth's surface are slowly changing their level.
12. In what way does the action of the earth's internal heat
tend to counteract the general lowering of level caused by the
destructive action of air, rain, frosts, rivers, glaciers, and the
sea?
13. Under what circumstances were the rocks of most of our
hills and valleys formed?
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Literature Primer,.
ENGLISH GRAMMAR. R. Morris.
ENGLISH LITERAl*UR£. Stopford A. Bi
PHILOLOGY. J. PeUe.
CLASSICAL GEOGRAPHY. M. F. Toxcr,
SHAB;ESP£AR£. £. Dowden.
STUDIES IN BRYANT. J. Alden.
GREEK LITERATURE. R. C Jebb.
NGLISH GRAMMAR EXERCISES. R. MorriiP.
UOWSR* W. E. Gladstone.
ENGLISH COMPOSITION. J. Nichol.
n M^PT.K JON <^v CO.
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